igb: use new eth_get_headlen interface

Update igb to drop the igb_get_headlen function in favor of eth_get_headlen.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Acked-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>