udp: add support for UDP_GRO cmsg

When UDP GRO is enabled, the UDP_GRO cmsg will carry the ingress
datagram size. User-space can use such info to compute the original
packets layout.

Change-Id: If86f1e5119cd3c2836ee8b93cfb00b7df7d15206
CRs-Fixed: 2351793
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next.git


Git-commit: bcd1665e3569b0a6f569514f023a41fc7df0b4a3
Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>