Phonet: fix mutex imbalance

From: Rémi Denis-Courmont <remi.denis-courmont@nokia.com>

port_mutex was unlocked twice.
Signed-off-by: Rémi Denis-Courmont <remi.denis-courmont@nokia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/phonet/socket.c

@@ -407,7 +407,6 @@ int pn_sock_get_port(struct sock *sk, unsigned short sport)
 	return -EADDRINUSE;
 
 found:
-	mutex_unlock(&port_mutex);
 	pn->sobject = pn_object(pn_addr(pn->sobject), sport);
 	return 0;
 }