netinet: provide "at offset" variant of the in_delayed_cksum() API

The need for such a variant comes from the fact that we need to
re-calculate checksum aftet ng_nat(4) transformations while getting
mbufs from the layer 2 (ethernet) directly.

Reviewed by: markj, tuexen
Approved by: tuexen
Sponsored by: Sippy Software, Inc.
Differential Revision: https://reviews.freebsd.org/D49677
MFC After: 2 weeks