All of libbpf errors are negative, except this one. Fix it.
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
 
        if (!obj->efile.strtabidx || obj->efile.strtabidx >= idx) {
                pr_warning("Corrupted ELF file: index of strtab invalid\n");
-               return LIBBPF_ERRNO__FORMAT;
+               return -LIBBPF_ERRNO__FORMAT;
        }
        if (btf_data) {
                obj->btf = btf__new(btf_data->d_buf, btf_data->d_size);