net: annotate sk->sk_err write from do_recvmmsg()

do_recvmmsg() can write to sk->sk_err from multiple threads.

As said before, many other points reading or writing sk_err
need annotations.

Fixes: 34b88a68f2 ("net: Fix use after free in the recvmmsg exit path")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/socket.c

@@ -2911,7 +2911,7 @@ static int do_recvmmsg(int fd, struct mmsghdr __user *mmsg,
 		 * error to return on the next call or if the
 		 * app asks about it using getsockopt(SO_ERROR).
 		 */
-		sock->sk->sk_err = -err;
+		WRITE_ONCE(sock->sk->sk_err, -err);
 	}
 out_put:
 	fput_light(sock->file, fput_needed);