pf: distinguish forwarding and output cases for pf_refragment6()

Re-introduce PFIL_FWD, because pf's pf_refragment6() needs to know if
we're ip6_forward()-ing or ip6_output()-ing.

ip6_forward() relies on m->m_pkthdr.rcvif, at least for link-local
traffic (for in6_get_unicast_scopeid()). rcvif is not set for locally
generated traffic (e.g. from icmp6_reflect()), so we need to call the
correct output function.

Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revisi: https://reviews.freebsd.org/D39061