While doing the initial SACK retransmission segment while heavily cwnd
constrained, tcp_ouput can erraneously 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. In this case,
cwnd is incorrectly inflated, leading to the inappropriate transmission of
all segments above snd_una.