pf: drop IPv6 packets built from overlapping fragments in pf reassembly

The reassembly state will be dropped after timeout, all related
fragments are dropped until that. This is conforming to RFC 5722.

• Sort pf_fragment fields while there.
• If the fr_queue is empty, we had overlapping fragments, don't add new ones.
• If we detect overlapping IPv6 fragments, flush the fr_queue and drop all fragments immediately.
• Rearrange debug output, to make clear what happens.
• An IPv4 fragment that is totaly overlapped does not inclease the bad fragment counter.
• Put an KASSERT into pf_isfull_fragment() to make sure that the fr_queue is never emtpy there.

discussed with Fernando Gont; ok henning@

Obtained from: OpenBSD, bluhm <bluhm@openbsd.org>, 8b45f36762
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit 6a3266f72e437aecf3edcfb8aa919466b270d548)