tcp: warn on negative reordering values

Commit bafbb9c73241 ("tcp: eliminate negative reordering
in tcp_clean_rtx_queue") fixes an issue for negative
reordering metrics.

To be resilient to such errors, warn and return
when a negative metric is passed to tcp_update_reordering().

Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -887,6 +887,9 @@ static void tcp_update_reordering(struct sock *sk, const int metric,
 	struct tcp_sock *tp = tcp_sk(sk);
 	int mib_idx;
 
+	if (WARN_ON_ONCE(metric < 0))
+		return;
+
 	if (metric > tp->reordering) {
 		tp->reordering = min(sysctl_tcp_max_reordering, metric);