Add AF_XDP samples

[akamath32]

First of all thank you for this project. I had a hard time building the bpf/XDP samples in the kernel tree and this project worked much more smoothly.

This PR is adding the sample xdpsock from the kernel tree which uses the new AF_XDP socket layer.

[netoptimizer]

Hmmm... I'm not sure it is a good idea to duplicate kernel sample xdpsock here... because that means that I/we have to maintain it it two places.

I know that @magnus-karlsson are preparing a patchset that improves this sample, and I don't want to have to backport that work...