i40e: Rx checksum offload for VXLAN

This implements receive offload for VXLAN for i40e.  The hardware
supports checksum offload/verification of the inner/outer header.

Change-Id: I450db300af6713f2044fef1191a0d1d294c13369
Signed-off-by: Joseph Gasparakis <joseph.gasparakis@intel.com>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>