selftests: add dummy xdp test helper

This trivial XDP program does nothing, but will be used by the
next patch to test the GRO path in a net namespace, leveraging
the veth XDP implementation.

It's added here, despite its 'net' usage, to avoid the duplication
of the llc-related makefile boilerplate.

rfc v3 -> v1:
 - move the helper implementation into the bpf directory, don't
   touch udpgso_bench_rx

rfc v2 -> rfc v3:
 - move 'x' option handling here

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>