selftests/bpf: Add BPF XDP link selftests

Add selftest validating all the attachment logic around BPF XDP link. Test
also link updates and get_obj_info() APIs.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200722064603.3350758-9-andriin@fb.com