pfctl: fix incorrect mask on dynamic address

A PF rule using an IPv4 address followed by an IPv6 address and then a
dynamic address, e.g. "pass from {192.0.2.1 2001:db8::1} to (pppoe0)",
will have an incorrect /32 mask applied to the dynamic address.

MFC after: 3 weeks
Obtained from: OpenBSD
See also: https://ftp.openbsd.org/pub/OpenBSD/patches/5.6/common/007_pfctl.patch.sig
Sponsored by: Rubicon Communications, LLC ("Netgate")
Event: Oslo Hackathon at Modirum

(cherry picked from commit 7ce98cf2f87a22240b66e4c38fd887431a25bf7d)