pf: only check MTU for IPv6 packets when forwarding

When the packets are generated locally (i.e. PFIL_FWD is not set) we
might generate overly large packets and rely on the NIC to fragment it
for us. In that case we'd reject a valid packet.

Reported by: Herbert J. Skuhra <herbert@gojira.at>
Tested by: Herbert J. Skuhra <herbert@gojira.at>
Fixes: 54c62e3e5d8cd90c5571a1d4c8c5f062d580480e
Sponsored by: Rubicon Communications, LLC ("Netgate")