Recalculate checksum of normalised packet

Recalculate checksum of normalised packet

In 2011, henning@ removed fiddling with the ip checksum of normalised
packets in r1.131 of sys/net/pf_norm.c. Rationale was that the checksum
is always recalculated in all output paths anyway. In 2016, procter@
reintroduced checksum modification to preserve end-to-end checksums in
r1.189 of sys/net/pf_norm.c. Likely soomewhere in that timeslot checksum
recalculation of normalised packets was broken.

With input from bluhm@.

OK sashan@, bluhm@