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