libbpf: Fix map index used in error message

The error message emitted by bpf_object__init_user_btf_maps() was using the
wrong section ID.

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20200819110534.9058-1-toke@redhat.com

tools/lib/bpf/libbpf.c

@@ -2264,7 +2264,7 @@ static int bpf_object__init_user_btf_maps(struct bpf_object *obj, bool strict,
 		data = elf_getdata(scn, NULL);
 	if (!scn || !data) {
 		pr_warn("failed to get Elf_Data from map section %d (%s)\n",
-			obj->efile.maps_shndx, MAPS_ELF_SEC);
+			obj->efile.btf_maps_shndx, MAPS_ELF_SEC);
 		return -EINVAL;
 	}