tcp: Correctly compute the TCP goodput in bits per second by using SEQ_SUB().
TCP sequence number differences should be computed using SEQ_SUB().
Differential Revision: https://reviews.freebsd.org/D35505
Reviewed by: rscheff@
Sponsored by: NVIDIA Networking
(cherry picked from commit f5766992c0d0491d571cb6d1c47f373314f527a8)