veth: add netpoll support

It is trivial to add netpoll support to veth, since
it is not a stacked device, we don't need to setup and
clean up netpoll.

Reported-by: Stefan Priebe <s.priebe@profihost.ag>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Neil Horman <nhorman@tuxdriver.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>