pfsync: support deferring IPv6 packets

When we send out a deferred packet we must make sure to call
ip6_output() for IPv6 packets. If not we might end up attempting to
ip_fragment() an IPv6 packet, which could lead to us reading outside of
the mbuf.

PR: 268246
Reviewed by: melifaro, zlei
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D38586