vlan: add 802.1q netpoll support

Add netpoll support to 802.1q vlan devices.  Based on the netpoll support
in the bridging code.  Tested on a forced_eth device with netconsole.

Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>
Signed-off-by: David S. Miller <davem@davemloft.net>