[TCP]: Make invariant check complain about invalid sacked_out

Earlier resolution for NewReno's sacked_out should now keep
it small enough for this to become invariant-like check.
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -2504,11 +2504,8 @@ tcp_fastretrans_alert(struct sock *sk, int pkts_acked, int flag)
 				    (tcp_fackets_out(tp) > tp->reordering));
 	int fast_rexmit = 0;
 
-	/* Some technical things:
-	 * 1. Reno does not count dupacks (sacked_out) automatically. */
-	if (!tp->packets_out)
+	if (WARN_ON(!tp->packets_out && tp->sacked_out))
 		tp->sacked_out = 0;
-
 	if (WARN_ON(!tp->sacked_out && tp->fackets_out))
 		tp->fackets_out = 0;