If a nhop gets an interface event, revalidate the nhops and
immediately try to recompile existing nexthop groups by
replacing unreachable nexthops with reachable ones.
If none are available, recompile them back to
their normal position in nexthop group slots.
Relnotes: yes
FINALLY TRUE ECMP WITHOUT EXTRA ALLOCATION!
in comparison to other OSes, we don't simply remove unreachable routes.
Now instead we replace them with reachable one.
We also don't want to walk through RIB to filter out ECMP ones.
We directly recompile nhgrps.