bpf: Implement BPF XDP link-specific introspection APIs

Implement XDP link-specific show_fdinfo and link_info to emit ifindex.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200722064603.3350758-7-andriin@fb.com