macvtap: Close a race between macvtap_open and macvtap_dellink.

There is a small window in macvtap_open between looking up a
networking device and calling macvtap_set_queue in which
macvtap_del_queues called from macvtap_dellink.   After
calling macvtap_del_queues it is totally incorrect to
allow macvtap_set_queue to proceed so prevent success by
reporting that all of the available queues are in use.
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/macvtap.c

@@ -231,6 +231,8 @@ static void macvtap_del_queues(struct net_device *dev)
 		}
 	}
 	BUG_ON(vlan->numvtaps != 0);
+	/* guarantee that any future macvtap_set_queue will fail */
+	vlan->numvtaps = MAX_MACVTAP_QUEUES;
 	spin_unlock(&macvtap_lock);
 
 	synchronize_rcu();