driver: macvtap: Unregister netdev rx_handler if macvtap_newlink fails

The macvtap_newlink registers the netdev rx_handler firstly, but it
does not unregister the handler if macvlan_common_newlink failed.

Signed-off-by: Gao Feng <fgao@ikuai8.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>