bgpd: nht unresolved with global address next-hop

bgpd: nht unresolved with global address next-hop

When bgp peers with ipv6 link local addresses, it may receive a
BGP update with next-hop containing both LL and GA information.
By default, nexthop tracking applies to GA, and ignores presence
of LL, when both addresses are present. This is a problem for
resolving GA as next-hop as the next-hop information can be solved
by using the LL address only.

The solution consists in defaulting the nexthop ipv6 choice to LL
when available, and moving back to GA if a route-map is locally
configured at inbound.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>
(cherry picked from commit 17ef5a934355a718caf64f2fd5cac5e252977890)