Merge branch 'bpf-btf-id'

Martin KaFai Lau says:

====================
This series introduces BTF ID which is exposed through
the new BPF_BTF_GET_FD_BY_ID cmd, new "struct bpf_btf_info"
and new members in the "struct bpf_map_info".

Please see individual patch for details.
====================

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