pf: Fix unaligned checksum updates

The algorithm we use to update checksums only works correctly if the
updated data is aligned on 16-bit boundaries (relative to the start of
the packet).

Import the OpenBSD fix for this issue.

PR: 240416
Obtained from: OpenBSD
MFC after: 1 week
Reviewed by: tuexen (previous version)
Differential Revision: https://reviews.freebsd.org/D27696