routing: Replace unreachable nhops in nhgrp

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.

Reviewed by: glebius
Discussed with: markj
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D57389