In commit 8d8fc29d
(netpoll: disable netpoll when enslave a device), we automatically
disable netpoll when the underlying device is being enslaved,
we also need to prevent people from setuping netpoll on
devices that are already enslaved.
Signed-off-by: NWANG Cong <amwang@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>