mbuf: PACKET_TAG_PF should not be persistent

We should clear firewall tags on loopback, icmp reflection, or if_epair
transmission. Left over tags can produce unexpected behaviour,
especially on if_epair where a and b interfaces can be in different
vnets, and have different firewall policies set.

MFC after: 3 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D32664