pf: send ICMP destination unreachable fragmentation needed when appropriate

Just like we do for IPv6, generate an ICMP fragmentation needed packet if we're
going to need fragmenation for IPv4 as well (i.e. DF is set). Do so before full
processing, so we generate it with pre-NAT addreses, just as we do for IPv6.

Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D48805