igb: make ethtool use core xmit map and free functionality

This change adds a clean_rx/tx_irq type function call to the ethtool loopback
testing which allows us to test the core transmit and receive functionality in
the driver.

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>