[TCP]: Catch skb with S+L bugs earlier

SACKED_ACKED and LOST are mutually exclusive with SACK, thus
having their sum larger than packets_out is bug with SACK.
Eventually these bugs trigger traps in the tcp_clean_rtx_queue
with SACK but it's much more informative to do this here.

Non-SACK TCP, however, could get more than packets_out duplicate
ACKs which each increment sacked_out, so it makes sense to do
this kind of limitting for non-SACK TCP but not for SACK enabled
one. Perhaps the author had the opposite in mind but did the
logic accidently wrong way around? Anyway, the sacked_out
incrementer code for non-SACK already deals this issue before
calling sync_left_out so this trapping can be done
unconditionally.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -736,9 +736,7 @@ static inline __u32 tcp_current_ssthresh(const struct sock *sk)
 
 static inline void tcp_sync_left_out(struct tcp_sock *tp)
 {
-	if (tp->rx_opt.sack_ok &&
-	    (tp->sacked_out >= tp->packets_out - tp->lost_out))
-		tp->sacked_out = tp->packets_out - tp->lost_out;
+	BUG_ON(tp->sacked_out + tp->lost_out > tp->packets_out);
 	tp->left_out = tp->sacked_out + tp->lost_out;
 }