pf: Fix forwarding detection

If the direction is not PF_OUT we can never be forwarding. Some input packets
have rcvif != ifp (looped back packets), which lead us to ip6_forward() inbound
packets, causing panics.

Equally, we need to ensure that packets were really received and not locally
generated before trying to ip6_forward() them.

Differential Revision: https://reviews.freebsd.org/D2286
Approved by: gnn(mentor)