igb: Allocate rings seperately instead of as a block

This change makes it so that the rings are allocated seperately.  As a
result we can allocate them on seperate nodes at some point in the future
if we so desire.

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>