Use UDP len when calculating UDP checksums

The length of the IP payload is normally equal to the UDP length, UDP Options
(draft-ietf-tsvwg-udp-options-02) suggests using the difference between IP
length and UDP length to create space for trailing data.

Correct checksum length calculation to use the UDP length rather than the IP
length when not offloading UDP checksums.

Approved by: jtl (mentor)
Differential Revision: https://reviews.freebsd.org/D15222