tcp: Fix a data-race around sysctl_tcp_nometrics_save.

While reading sysctl_tcp_nometrics_save, it can be changed concurrently.
Thus, we need to add READ_ONCE() to its reader.

Fixes: 1da177e ("Linux-2.6.12-rc2")
Signed-off-by: Kuniyuki Iwashima <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

Author: q2ven

net/ipv4/tcp_metrics.c

@@ -329,7 +329,7 @@ void tcp_update_metrics(struct sock *sk)
 	int m;
 
 	sk_dst_confirm(sk);
-	if (net->ipv4.sysctl_tcp_nometrics_save || !dst)
+	if (READ_ONCE(net->ipv4.sysctl_tcp_nometrics_save) || !dst)
 		return;
 
 	rcu_read_lock();