i40e: Add support for ATR w/ IPv6 extension headers

This patch updates the code for determining the L4 protocol and L3 header
length so that when IPv6 extension headers are being used we can determine
the offset and type of the L4 protocol.

Signed-off-by: Alexander Duyck <aduyck@mirantis.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>