ip_output: pass PFIL_FWD in the slow path

If we take the slow path for forwarding we should still tell our
firewalls (hooked through pfil(9)) that we're forwarding. Pass the
ip_output() flags to ip_output_pfil() so it can set the PFIL_FWD flag
when we're forwarding.

MFC after: 1 week
Sponsored by: Axiado