tcp: Fix sending of TCP segments with IP level options

When bringing in TCP over UDP support in
https://cgit.FreeBSD.org/src/commit/?id=9e644c23000c2f5028b235f6263d17ffb24d3605,
the length of IP level options was considered when locating the
transport header. This was incorrect and is fixed by this patch.

X-MFC with: https://cgit.FreeBSD.org/src/commit/?id=9e644c23000c2f5028b235f6263d17ffb24d3605
Reviewed by: markj, rscheff
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D30358

(cherry picked from commit 500eb6dd80404ea512e31a8f795c73cb802c9c64)