Phonet: use atomic for packet TX window

GPRS TX flow control won't need to lock the underlying socket anymore.

Signed-off-by: Rémi Denis-Courmont <remi.denis-courmont@nokia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/phonet/socket.c b/net/phonet/socket.c
index c75aa5c..ada2a35 100644
--- a/net/phonet/socket.c
+++ b/net/phonet/socket.c
@@ -227,7 +227,7 @@
 	if (!mask && sk->sk_state == TCP_CLOSE_WAIT)
 		return POLLHUP;
 
-	if (sk->sk_state == TCP_ESTABLISHED && pn->tx_credits)
+	if (sk->sk_state == TCP_ESTABLISHED && atomic_read(&pn->tx_credits))
 		mask |= POLLOUT | POLLWRNORM | POLLWRBAND;
 
 	return mask;