igb: add a flags value to the ring

This patch adds a flags value to the ring that cleans up some of the last
remaining items from the ring in order to help seperate it from the adapter
struct.  By implementing these flags it becomes possible for different rings
to support different functions such as rx checksumming.

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>