When the TCP sequence number subtracted is greater than 2**32 minus
the limit, or 2**31 minus the limit, the use of unsigned long as an
intermediate variable, may result in an incorrect retransmit length
computation on all 64-bit platforms in the TCP stack.
While at it create a helper macro to facilitate the computation of
the difference between TCP sequence numbers.
MFC after: 3 days
Sponsored by: NVIDIA Networking