selftests: udp gso benchmark

Send udp data between a source and sink, optionally with udp gso.
The two processes are expected to be run on separate hosts.

A script is included that runs them together over loopback in a
single namespace for functionality testing.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>