libbpf: Add internal helper to load BTF data by FD

Add a btf_get_from_fd() helper, which constructs struct btf from in-kernel BTF
data by FD. This is used for loading module BTFs.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20201203204634.1325171-4-andrii@kernel.org

src/libbpf_internal.h

@@ -155,6 +155,7 @@ int bpf_object__section_size(const struct bpf_object *obj, const char *name,
 			     __u32 *size);
 int bpf_object__variable_offset(const struct bpf_object *obj, const char *name,
 				__u32 *off);
+struct btf *btf_get_from_fd(int btf_fd, struct btf *base_btf);
 
 struct btf_ext_info {
 	/*