ospfd: make ECMP nexthop order deterministic

ospfd: make ECMP nexthop order deterministic

The order of ECMP nexthops currently depends on whatever order the
pqueue code returns the vertices in, which is essentially random since
they compare as equal.  While this shouldn't cause issues normally, it
is nondeterministic and causes the ldp-topo1 test to fail when the
ordering comes up different.  Also, nondeterministic behaviour is not a
nice thing to have here in general.

Just sort by nexthop address;  realistic numbers of ECMP nexthops should
hopefully not make this a performance issue.  (Also, nexthops should be
hot in the caches here.)

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>