igb: add new data structure for handling interrupts and NAPI

Add a new igb_q_vector data structure to handle interrupts and NAPI.  This
helps to abstract the rings away from the adapter struct.  In addition it
allows for a bit of consolidation since a tx and rx ring can share a
q_vector.

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>