igb: Refactor clean_rx_irq to reduce overhead and improve performance

This change is meant to be a general cleanup and performance improvement
for clean_rx_irq.  The previous patch should have updated the allocation so
that the rings can be treated as read-only within the clean_rx_irq
function.  In addition I am re-ordering the operations such that several
goals are accomplished including reducing the overhead for packet
accounting, reducing the number of items on the stack, and improving
overall performance.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>