Similar to how the IPv4 code will reject an IPv6 LB group,
we must ignore IPv4 LB groups when looking up an IPv6
listening socket. If this is not done, a port only match
may return an IPv4 socket, which causes problems (like
sending IPv6 packets with a hopcount of 0, making them unrouteable).
Thanks to rrs for all the work to diagnose this.
Approved by: re (rgrimes)
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D16899