tcp: fix cwnd restricted SACK retransmission loop
While doing the initial SACK retransmission segment while heavily cwnd
constrained, tcp_ouput can erroneously send out the entire sendbuffer
again. This may happen after an retransmission timeout, which resets
snd_nxt to snd_una while the SACK scoreboard is still populated.
Reviewed By: tuexen, transport
MFC after: 3 days
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D36637