[TCP]: Use S+L catcher only with SACK for now

TCP has a transitional state when SACK is not in use during
which this invariant is temporarily broken. Without SACK,
tcp_clean_rtx_queue does not decrement sacked_out. Therefore
calls to tcp_sync_left_out before sacked_out is again
corrected by tcp_fastretrans_alert can trigger this trap as
sacked_out still has couple of segments that are already out
of window.
Signed-off-by: NIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -736,7 +736,8 @@ static inline __u32 tcp_current_ssthresh(const struct sock *sk)
 
 static inline void tcp_sync_left_out(struct tcp_sock *tp)
 {
-	BUG_ON(tp->sacked_out + tp->lost_out > tp->packets_out);
+	BUG_ON(tp->rx_opt.sack_ok &&
+	       (tp->sacked_out + tp->lost_out > tp->packets_out));
 	tp->left_out = tp->sacked_out + tp->lost_out;
 }