ipvlan: use netdev_is_rx_handler_busy instead of checking specific type

IPvlan checks if the master device is already used by checking a
specific device (here it's macvlan device). This is technically not
sufficient and it should just ensure the rx_handler is busy or not.
This would be a super check that includes macvlan and any other that
has already registered rx-handler.

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