libbpf: Allow loading empty BTFs

Empty BTFs do come up (e.g., simple kernel modules with no new types and
strings, compared to the vmlinux BTF) and there is nothing technically wrong
with them. So remove unnecessary check preventing loading empty BTFs.

Fixes: d8123624 ("libbpf: Fix BTF data layout checks and allow empty BTF")
Reported-by: NChristopher William Snowhill <chris@kode54.net>
Signed-off-by: NAndrii Nakryiko <andrii@kernel.org>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210110070341.1380086-2-andrii@kernel.org

tools/lib/bpf/btf.c

@@ -240,11 +240,6 @@ static int btf_parse_hdr(struct btf *btf)
 	}
 
 	meta_left = btf->raw_size - sizeof(*hdr);
-	if (!meta_left) {
-		pr_debug("BTF has no data\n");
-		return -EINVAL;
-	}
-
 	if (meta_left < hdr->str_off + hdr->str_len) {
 		pr_debug("Invalid BTF total size:%u\n", btf->raw_size);
 		return -EINVAL;