udp: add udp gso

Implement generic segmentation offload support for udp datagrams. A
follow-up patch adds support to the protocol stack to generate such
packets.

UDP GSO is not UFO. UFO fragments a single large datagram. GSO splits
a large payload into a number of discrete UDP datagrams.

The implementation adds a GSO type SKB_UDP_GSO_L4 to differentiate it
from UFO (SKB_UDP_GSO).

IPPROTO_UDPLITE is excluded, as that protocol has no gso handler
registered.

Change-Id: Iff5e2f40ba816b4ee6675fa5a8f47d78c2ba91cb
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Git-commit: ee80d1ebe5ba7f4bd74959c873119175a4fc08d3
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next.git


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