i40e/i40evf: Consolidate all header changes into TSO function

This patch goes through and pulls all of the spots where we were updating
either the TCP or IP checksums in the TSO and checksum path into the TSO
function.  The general idea here is that we should only be updating the
header after we verify we have completed a skb_cow_head check to verify the
head is writable.

One other advantage to doing this is that it makes things much more
obvious.  For example, in the case of IPv6 there was one spot where the
offset of the IPv4 header checksum was being updated which is obviously
incorrect.

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>