Reject IPv4 SO_REUSEPORT_LB groups when looking up an IPv6 listening socket

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