bpf, doc: howto use/run the BPF selftests

I always forget howto run the BPF selftests. Thus, lets add that info
to the QA document.

Documentation was based on Cilium's documentation:
 http://cilium.readthedocs.io/en/latest/bpf/#verifying-the-setup



Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>