tcp: fix reverting of spurious timeouts (RTO)

One variable that became critical to correctly calculate
the cwnd during limited transmit was not properly reverted
on detection of spurious timeouts.

PR: 282605
Reviewed By: cc, tuexen, transport
MFC after: 3 days
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D48652