ng_nat: fix potential crash when attaching to L2 directly

Fix potential crash in the ng_nat module when attaching directly
to the layer 2 (ethernet) while calculating TCP checksum.

The issue is due to in_delayed_cksum() expecting to access IP
header at the offset 0 from the mbuf start, while if we are
attached to the L2 directly, the IP header at going to be at the
certain offset.

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