zebra: disallow resolution to duplicate nexthops

zebra: disallow resolution to duplicate nexthops

Disallow the resolution to nexthops that are marked duplicate.
When we are resolving to an ecmp group, it's possible this
group has duplicates.

I found this when I hit a bug where we can have groups resolving
to each other and cause the resolved->next->next pointer to increase
exponentially. Sufficiently large ecmp and zebra will grind to a hault.

Like so:

```
D>  4.4.4.14/32 [150/0] via 1.1.1.1 (recursive), weight 1, 00:00:02
  *                       via 1.1.1.1, dummy1 onlink, weight 1, 00:00:02
                        via 4.4.4.1 (recursive), weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                        via 4.4.4.2 (recursive), weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                        via 4.4.4.3 (recursive), weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                        via 4.4.4.4 (recursive), weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                        via 4.4.4.5 (recursive), weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                        via 4.4.4.6 (recursive), weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                        via 4.4.4.7 (recursive), weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                        via 4.4.4.8 (recursive), weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                        via 4.4.4.9 (recursive), weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                        via 4.4.4.10 (recursive), weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                        via 4.4.4.11 (recursive), weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                        via 4.4.4.12 (recursive), weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                        via 4.4.4.13 (recursive), weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                        via 4.4.4.15 (recursive), weight 1, 00:00:02
                          via 1.1.1.1, dummy1 onlink, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1 onlink, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                        via 4.4.4.16 (recursive), weight 1, 00:00:02
                          via 1.1.1.1, dummy1 onlink, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
                          via 1.1.1.1, dummy1, weight 1, 00:00:02
D>  4.4.4.15/32 [150/0] via 1.1.1.1 (recursive), weight 1, 00:00:09
  *                       via 1.1.1.1, dummy1 onlink, weight 1, 00:00:09
                        via 4.4.4.1 (recursive), weight 1, 00:00:09
                          via 1.1.1.1, dummy1, weight 1, 00:00:09
                        via 4.4.4.2 (recursive), weight 1, 00:00:09
                          via 1.1.1.1, dummy1, weight 1, 00:00:09
                        via 4.4.4.3 (recursive), weight 1, 00:00:09
                          via 1.1.1.1, dummy1, weight 1, 00:00:09
                        via 4.4.4.4 (recursive), weight 1, 00:00:09
                          via 1.1.1.1, dummy1, weight 1, 00:00:09
                        via 4.4.4.5 (recursive), weight 1, 00:00:09
                          via 1.1.1.1, dummy1, weight 1, 00:00:09
                        via 4.4.4.6 (recursive), weight 1, 00:00:09
                          via 1.1.1.1, dummy1, weight 1, 00:00:09
                        via 4.4.4.7 (recursive), weight 1, 00:00:09
                          via 1.1.1.1, dummy1, weight 1, 00:00:09
                        via 4.4.4.8 (recursive), weight 1, 00:00:09
                          via 1.1.1.1, dummy1, weight 1, 00:00:09
                        via 4.4.4.9 (recursive), weight 1, 00:00:09
                          via 1.1.1.1, dummy1, weight 1, 00:00:09
                        via 4.4.4.10 (recursive), weight 1, 00:00:09
                          via 1.1.1.1, dummy1, weight 1, 00:00:09
                        via 4.4.4.11 (recursive), weight 1, 00:00:09
                          via 1.1.1.1, dummy1, weight 1, 00:00:09
                        via 4.4.4.12 (recursive), weight 1, 00:00:09
                          via 1.1.1.1, dummy1, weight 1, 00:00:09
                        via 4.4.4.13 (recursive), weight 1, 00:00:09
                          via 1.1.1.1, dummy1, weight 1, 00:00:09
                        via 4.4.4.14 (recursive), weight 1, 00:00:09
                          via 1.1.1.1, dummy1, weight 1, 00:00:09
                        via 4.4.4.16 (recursive), weight 1, 00:00:09
                          via 1.1.1.1, dummy1 onlink, weight 1, 00:00:09
                          via 1.1.1.1, dummy1, weight 1, 00:00:09
                          via 1.1.1.1, dummy1, weight 1, 00:00:09
                          via 1.1.1.1, dummy1, weight 1, 00:00:09
                          via 1.1.1.1, dummy1, weight 1, 00:00:09
                          via 1.1.1.1, dummy1, weight 1, 00:00:09
                          via 1.1.1.1, dummy1, weight 1, 00:00:09
                          via 1.1.1.1, dummy1, weight 1, 00:00:09
                          via 1.1.1.1, dummy1, weight 1, 00:00:09
                          via 1.1.1.1, dummy1, weight 1, 00:00:09
                          via 1.1.1.1, dummy1, weight 1, 00:00:09
                          via 1.1.1.1, dummy1, weight 1, 00:00:09
                          via 1.1.1.1, dummy1, weight 1, 00:00:09
                          via 1.1.1.1, dummy1, weight 1, 00:00:09
                          via 1.1.1.1, dummy1, weight 1, 00:00:09
                          via 1.1.1.1, dummy1, weight 1, 00:00:09
D>  4.4.4.16/32 [150/0] via 1.1.1.1 (recursive), weight 1, 00:00:19
  *                       via 1.1.1.1, dummy1 onlink, weight 1, 00:00:19
                        via 4.4.4.1 (recursive), weight 1, 00:00:19
                          via 1.1.1.1, dummy1, weight 1, 00:00:19
                        via 4.4.4.2 (recursive), weight 1, 00:00:19

...............
................

and on...

```

You can repro the above via:

```
kernel routes:

1.1.1.1 dev dummy1 scope link

4.4.4.0/24 via 1.1.1.1 dev dummy1

==============================

config:

nexthop-group doof
 nexthop 1.1.1.1
 nexthop 4.4.4.1
 nexthop 4.4.4.10
 nexthop 4.4.4.11
 nexthop 4.4.4.12
 nexthop 4.4.4.13
 nexthop 4.4.4.14
 nexthop 4.4.4.15
 nexthop 4.4.4.16
 nexthop 4.4.4.2
 nexthop 4.4.4.3
 nexthop 4.4.4.4
 nexthop 4.4.4.5
 nexthop 4.4.4.6
 nexthop 4.4.4.7
 nexthop 4.4.4.8
 nexthop 4.4.4.9
!

===========================

Then use sharpd to install 4.4.4.16 -> 4.4.4.1 pointing to that nexthop
group in decending order.
```

With these changes it prevents the growing ecmp above by disallowing
duplicates to be in the resolution decision. These nexthops are not
installed anyways so why should we be resolving to them?

Signed-off-by: Stephen Worley <sworley@nvidia.com>