igb: add NAPI Rx queue support

Update the NAPI implementation to use the new napi_struct infrstructure.
This removes the need of multiple net_device structs to implement a
multiqueue NAPI.

Signed-off-by: PJ Waskiewicz <peter.p.waskiewicz.jr.@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>