tcp: annotate data-race around queue->synflood_warned

Annotate the lockless read of queue->synflood_warned.

Following xchg() has the needed data-race resolution.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -6841,7 +6841,7 @@ static bool tcp_syn_flood_action(const struct sock *sk, const char *proto)
 #endif
 		__NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPREQQFULLDROP);
 
-	if (!queue->synflood_warned && syncookies != 2 &&
+	if (!READ_ONCE(queue->synflood_warned) && syncookies != 2 &&
 	    xchg(&queue->synflood_warned, 1) == 0) {
 		if (IS_ENABLED(CONFIG_IPV6) && sk->sk_family == AF_INET6) {
 			net_info_ratelimited("%s: Possible SYN flooding on port [%pI6c]:%u. %s.\n",