tcp: fix computation of offset

Only update the offset if actually retransmitting from the
scoreboard. If not done correctly, this may result in
trying to (re)-transmit data not being being in the socket
buffe and therefore resulting in a panic.

PR: 264257
PR: 263445
PR: 260393
Reviewed by: rscheff@
MFC after: 3 days
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D36626