Fix ICMPv6 redirects

icmp6_redirect_input() validates that a redirect packet came from the
current gateway for the respective destination. To do this, it compares
the source address, which has an embedded scope zone id, to the next-hop
address, which does not. If the address is link-local, which should be
the case, the comparison fails and the redirect is ignored.

Insert the scope zone id into the next-hop address so the comparison
is accurate.

Unsurprisingly, this fixes 35 UNH IPv6 conformance test cases.

Submitted by: Farrell Woods <Farrell_Woods@Dell.com> (initial revision)
Reviewed by: ae melifaro dab
MFC after: 1 week
Relnotes: yes
Sponsored by: Dell EMC
Differential Revision: https://reviews.freebsd.org/D14254