net: remove duplicate code.

netdev_rx_handler_register() checks to see if the handler is already
busy which was recently separated into netdev_is_rx_handler_busy(). So
use the same function inside register() to avoid code duplication.
Essentially this change should be a no-op

Signed-off-by: Mahesh Bandewar <maheshb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -3961,9 +3961,7 @@ int netdev_rx_handler_register(struct net_device *dev,
 			       rx_handler_func_t *rx_handler,
 			       void *rx_handler_data)
 {
-	ASSERT_RTNL();
-
-	if (dev->rx_handler)
+	if (netdev_is_rx_handler_busy(dev))
 		return -EBUSY;
 
 	/* Note: rx_handler_data must be set before rx_handler */