pf: deduplicate IPv4 and IPv6 code that handles fragments

Deduplicate IPv4 and IPv6 code that handles fragments that have not
been reassembled by normalization.
ok henning claudio

Obtained from: OpenBSD, bluhm <bluhm@openbsd.org>, ddd2494207
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D46934