pf: Fix IP checksum on reassembly

If we reassemble a packet we modify the IP header (to set the length and
remove the fragment offset information), but we failed to update the
checksum. On certain setups (mostly where we did not re-fragment again
afterwards) this could lead to us sending out packets with incorrect
checksums.

PR: 255432
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D30026

(cherry picked from commit 055c55abefbe19fe46a56894595af9c9dad7678c)