selftests/xsk: Verify correctness of XDP prog attach point

To prevent the case we had previously where for TEST_MODE_SKB, XDP prog
was attached in native mode, call bpf_xdp_query() after loading prog and
make sure that attach_mode is as expected.

Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/bpf/20220629143458.934337-4-maciej.fijalkowski@intel.com