i40e: check for possible incorrect ipv6 checksum

If the IPV6EXADD bit is set in the Rx descriptor status, there
was an optional extension header with an alternate IP address
detected.  The HW checksum offload doesn't handle the alternate
IP address correctly so likely comes up with the wrong answer.
Thus, if the bit is set we ignore the checksum offload value.

Change-ID: I70ff8d38cdcddccf44107691cae13d0c07c284c8
Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>