udp: Do not pass checksum as a parameter to GSO segmentation

This patch is meant to allow us to avoid having to recompute the checksum
from scratch and have it passed as a parameter.

Instead of taking that approach we can take advantage of the fact that the
length that was used to compute the existing checksum is included in the
UDP header.

Finally to avoid the need to invert the result we can just call csum16_add
and csum16_sub directly. By doing this we can avoid a number of
instructions in the loop that is handling segmentation.

Change-Id: I1a5a87a3322155bad04f4e05053067002625f532
CRs-Fixed: 2351793
Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Git-commit: 9a0d41b3598ff62ecb26661bbfb1d523586cdea3
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/davem/net.git


Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>