bpf: selftests: introduce bpf_xdp_{load,store}_bytes selftest

Introduce kernel selftest for new bpf_xdp_{load,store}_bytes helpers.
and bpf_xdp_pointer/bpf_xdp_copy_buf utility routines.

Acked-by: Toke Hoiland-Jorgensen <toke@redhat.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/2c99ae663a5dcfbd9240b1d0489ad55dea4f4601.1642758637.git.lorenzo@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>