selftests/bpf: switch to new libbpf XDP APIs

Switch to using new bpf_xdp_*() APIs across all selftests. Take
advantage of a more straightforward and user-friendly semantics of
old_prog_fd (0 means "don't care") in few places.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20220120061422.2710637-4-andrii@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>