tcp: Rewind erraneous RTO only while performing RTO retransmissions

Under rare circumstances, a spurious retranmission is
incorrectly detected and rewound, messing up various tcpcb values,
which can lead to a panic when SACK is in use.

Reviewed By: tuexen, chengc_netapp.com, #transport
MFC after:   3 days
Sponsored by:        NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D33979

(cherry picked from commit 68e623c3f0afb4c9b338046fd1d4476befd82324)