`in6_neighbor` cache doesn't properly account for link-local next-hops

[dkg]

Depending on how route advertisements are configured, it's possible that the IPv6 next-hop could be on a link-local address (i.e. fe80:*). Doing a lookup based on that address alone is a mistake, seince there could be multiple links on the local machine, and link-local addresses need to be identified per-interface, not per-machine.