bpftool: Use BTF field iterator in btfgen

Switch bpftool's code which is using libbpf-internal
btf_type_visit_type_ids() helper to new btf_field_iter functionality.

This makes bpftool code simpler, but also unblocks removing libbpf's
btf_type_visit_type_ids() helper completely.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Alan Maguire <alan.maguire@oracle.com>
Reviewed-by: Quentin Monnet <qmo@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20240605001629.4061937-5-andrii@kernel.org

tools/bpf/bpftool/gen.c

@@ -2379,15 +2379,6 @@ static int btfgen_record_obj(struct btfgen_info *info, const char *obj_path)
 	return err;
 }
 
-static int btfgen_remap_id(__u32 *type_id, void *ctx)
-{
-	unsigned int *ids = ctx;
-
-	*type_id = ids[*type_id];
-
-	return 0;
-}
-
 /* Generate BTF from relocation information previously recorded */
 static struct btf *btfgen_get_btf(struct btfgen_info *info)
 {
@@ -2467,10 +2458,15 @@ static struct btf *btfgen_get_btf(struct btfgen_info *info)
 	/* second pass: fix up type ids */
 	for (i = 1; i < btf__type_cnt(btf_new); i++) {
 		struct btf_type *btf_type = (struct btf_type *) btf__type_by_id(btf_new, i);
+		struct btf_field_iter it;
+		__u32 *type_id;
 
-		err = btf_type_visit_type_ids(btf_type, btfgen_remap_id, ids);
+		err = btf_field_iter_init(&it, btf_type, BTF_FIELD_ITER_IDS);
 		if (err)
 			goto err_out;
+
+		while ((type_id = btf_field_iter_next(&it)))
+			*type_id = ids[*type_id];
 	}
 
 	free(ids);