pfil: set PFIL_FWD for IPv4 forwarding

Just like we already do for IPv6 set the PFIL_FWD flag when we're forwarding
IPv4 traffic. This allows firewalls to make more precise decisions.

Reviewed by: glebius
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D48824