x25: decrement netdev reference counts on unload

x25 does not decrement the network device reference counts on module unload.
Thus unregistering any pre-existing interface after unloading the x25 module
hangs and results in

 unregister_netdevice: waiting for tap0 to become free. Usage count = 1

This patch decrements the reference counts of all interfaces in x25_link_free,
the way it is already done in x25_link_device_down for NETDEV_DOWN events.
Signed-off-by: NApollon Oikonomopoulos <apollon@noc.grnet.gr>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/x25/x25_link.c

@@ -394,6 +394,7 @@ void __exit x25_link_free(void)
 	list_for_each_safe(entry, tmp, &x25_neigh_list) {
 		nb = list_entry(entry, struct x25_neigh, node);
 		__x25_remove_neigh(nb);
+		dev_put(nb->dev);
 	}
 	write_unlock_bh(&x25_neigh_list_lock);
 }