packet: fix warnings in rollover lock contention

Avoid two xchg calls whose return values were unused, causing a
warning on some architectures.

The relevant variable is a hint and read without mutual exclusion.
This fix makes all writers hold the receive_queue lock.
Suggested-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/packet/af_packet.c

@@ -1301,17 +1301,12 @@ static int packet_rcv_has_room(struct packet_sock *po, struct sk_buff *skb)
 	int ret;
 	bool has_room;
 
-	if (po->prot_hook.func == tpacket_rcv) {
-		spin_lock(&po->sk.sk_receive_queue.lock);
-		ret = __packet_rcv_has_room(po, skb);
-		spin_unlock(&po->sk.sk_receive_queue.lock);
-	} else {
+	spin_lock_bh(&po->sk.sk_receive_queue.lock);
 	ret = __packet_rcv_has_room(po, skb);
-	}
-
 	has_room = ret == ROOM_NORMAL;
 	if (po->pressure == has_room)
-		xchg(&po->pressure, !has_room);
+		po->pressure = !has_room;
+	spin_unlock_bh(&po->sk.sk_receive_queue.lock);
 
 	return ret;
 }
@@ -3814,7 +3809,7 @@ static unsigned int packet_poll(struct file *file, struct socket *sock,
 			mask |= POLLIN | POLLRDNORM;
 	}
 	if (po->pressure && __packet_rcv_has_room(po, NULL) == ROOM_NORMAL)
-		xchg(&po->pressure, 0);
+		po->pressure = 0;
 	spin_unlock_bh(&sk->sk_receive_queue.lock);
 	spin_lock_bh(&sk->sk_write_queue.lock);
 	if (po->tx_ring.pg_vec) {