zebra: When processing route_entries ignore unusable routes

zebra: When processing route_entries ignore unusable routes

When zebra is processing routes to determine what to send
to the rib, suppose we have two routes (a) a route processed
earlier that none of it's nexthops were active and (b)
a route that has good nexthops but has a worse admin distance.

rib_process, would not relook at (a)'s nexthops because
the ROUTE_ENTRY_CHANGED flag was not true and it would
win when compared to (b) because it's admin distance
was better, leaving us with a state where we would
attempt and fail to install route (a) because it
was not valid.

Modify the code to consider the number of nexthops
we have as a determiner if we can use the route.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>