net/packet: annotate data race in packet_sendmsg()

There is a known race in packet_sendmsg(), addressed
in commit 32d3182cd2 ("net/packet: fix race in tpacket_snd()")

Now we have data_race(), we can use it to avoid a future KCSAN warning,
as syzbot loves stressing af_packet sockets :)

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/packet/af_packet.c

@@ -3034,10 +3034,13 @@ static int packet_sendmsg(struct socket *sock, struct msghdr *msg, size_t len)
 	struct sock *sk = sock->sk;
 	struct packet_sock *po = pkt_sk(sk);
 
-	if (po->tx_ring.pg_vec)
+	/* Reading tx_ring.pg_vec without holding pg_vec_lock is racy.
+	 * tpacket_snd() will redo the check safely.
+	 */
+	if (data_race(po->tx_ring.pg_vec))
 		return tpacket_snd(po, msg);
-	else
-		return packet_snd(sock, msg, len);
+
+	return packet_snd(sock, msg, len);
 }
 
 /*