igb: place a pointer to the netdev struct in the ring itself

This change adds a pointer to the netdev to the ring itself.  The idea being
at some point in the future it will be possible to support multiple netdevs
from a single adapter struct.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>