tcp: Correctly compute the retransmit length for all 64-bit platforms.

When the TCP sequence number subtracted is greater than 232 minus
the window size, or 231 minus the window size, the use of unsigned
long as an intermediate variable, may result in an incorrect retransmit
length computation on all 64-bit platforms.

While at it create a helper macro to facilitate the computation of
the difference between two TCP sequence numbers.

Differential Revision: https://reviews.freebsd.org/D35388
Reviewed by: rscheff
MFC after: 3 days
Sponsored by: NVIDIA Networking