During limited transmit, one additional segment should be
sent for each incoming ACK. As tcp_output considers the
SACKed bytes, adjust for this in the limited transmit
recalculation.
Also, in case snd_nxt is lower than snd_max (after an RTO),
let tcp_output only consider retransmissions from the SACK
scoreboard when again in fast recovery mode, or dupthresh
dupacks have been seen.
PR: 282605