pf: use AF_INET6 when comparing IPv6 addresses

Reviewed by: zlei
MFC after: 1 week
Obtained from: OpenBSD, henning <henning@openbsd.org>, cf7c0bf78a
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D46576

(cherry picked from commit 50ecaf1bd46a834c4fdaf483eb39df7bb62501e3)