tcp: fix TCP MD5 digest computation for TCP over UDP

Skip the UDP header for the computation. This is similar to
skipping IPv6 extension headers.

Reviewed by: cc, rscheff
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D40596

(cherry picked from commit 0fb0711dba76a32a2202d2f41d64aa1247b5e51d)