tcp: fix TCPSTAT accounting for SACK

Account for SACK retransmitted bytes once the actual length
is known. This prevents a call to tcp_maxseg() and prepares
for TSO support when transmitting from the SACK scoreboard.

Reviewed By: tuexen, transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D43447