tcp: calculate ssthresh on RTO according to RFC5681

per RFC5681, only adjust ssthresh on the initital
retransmission timeout. Since RTO often happens
during loss recovery, while cwnd no longer tracks
all data in flight, calculcate pipe properly.

Reviewed By: tuexen, transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D43768