Merge branch 'bpf-annotate-kv-pair'

Martin KaFai Lau says:

====================
The series allows the BPF loader to figure out the btf_key_id
and btf_value_id from a map's name by using BPF_ANNOTATE_KV_PAIR()
similarly as in iproute2 commit f823f36012fb ("bpf: implement
btf handling and map annotation").

It also removes the old 'typedef' way which requires two separate
typedefs (one for the key and one for the value).

By doing this, iproute2 and libbpf have one consistent way to
figure out the btf_key_type_id and btf_value_type_id for a map.

The first two patches are some prep/cleanup works. The last patch
introduces BPF_ANNOTATE_KV_PAIR.

v3:
- Replace some more *int*_t and u* usages with the
  equivalent __[su]* in btf.c

v2:
- Fix the incorrect '&&' check on container_type
  in bpf_map_find_btf_info().
- Expose the existing static btf_type_by_id() instead of
  creating a new one.
====================

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>