bpf: add support for btf_* fields in struct bpf_map_info

bpf: add support for btf_* fields in struct bpf_map_info

* bpf_attr.h (struct bpf_map_info_struct): Add btf_fd, btf_key_type_id,
and btf_value_type_id fields.
* bpf.c (print_bpf_map_info): Decode btf_fd, btf_key_type_id,
and btf_value_type_id fields introduced by Linux commits
v4.18-rc1~114^2~223^2~21^2~4 and v4.18-rc1~114^2~148^2~7^2~2.
* tests/bpf-obj_get_info_by_fd.c (main): Update expected output.