udp.7: add UDP_GRO

udp.7: add UDP_GRO

UDP_GRO was added in commit e20cf8d3f1f7
("udp: implement GRO for plain UDP sockets.")

    $ git describe --contains e20cf8d3f1f7
    linux/v5.0-rc1~129^2~379^2~8

Kernel source has example code in tools/testing/selftests/net/udpgro*

Per https://www.kernel.org/doc/man-pages/patches.html,
"Describe how you obtained the information in your patch":
I reviewed the relevant UDP_GRO patches.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Cc: <pabeni@redhat.com>
Cc: <netdev@vger.kernel.org>
[ alx: srcfix ]
Signed-off-by: Alejandro Colomar <alx@kernel.org>