libbpf: Return -ENODATA for missing btf section

As discussed before, return -ENODATA (No data available) would be more
meaningful than ENOENT (No such file or directory).
Suggested-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Changbin Du <changbin.du@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20221231151436.6541-1-changbin.du@gmail.com

tools/lib/bpf/btf.c

@@ -1004,7 +1004,7 @@ static struct btf *btf_parse_elf(const char *path, struct btf *base_btf,
 
 	if (!btf_data) {
 		pr_warn("failed to find '%s' ELF section in %s\n", BTF_ELF_SEC, path);
-		err = -ENOENT;
+		err = -ENODATA;
 		goto done;
 	}
 	btf = btf_new(btf_data->d_buf, btf_data->d_size, base_btf);