bpf: log struct/union attribute for forward type

Current btf internal verbose logger logs fwd type as
  [2] FWD A type_id=0
where A is the type name.

Commit 9d5f9f70 ("bpf: btf: fix struct/union/fwd types
with kind_flag") introduced kind_flag which can be used
to distinguish whether a forward type is a struct or
union.

Also, "type_id=0" does not carry any meaningful
information for fwd type as btf_type.type = 0 is simply
enforced during btf verification and is not used
anywhere else.

This commit changed the log to
  [2] FWD A struct
if kind_flag = 0, or
  [2] FWD A union
if kind_flag = 1.
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

kernel/bpf/btf.c

@@ -1595,12 +1595,18 @@ static s32 btf_fwd_check_meta(struct btf_verifier_env *env,
 	return 0;
 }
 
+static void btf_fwd_type_log(struct btf_verifier_env *env,
+			     const struct btf_type *t)
+{
+	btf_verifier_log(env, "%s", btf_type_kflag(t) ? "union" : "struct");
+}
+
 static struct btf_kind_operations fwd_ops = {
 	.check_meta = btf_fwd_check_meta,
 	.resolve = btf_df_resolve,
 	.check_member = btf_df_check_member,
 	.check_kflag_member = btf_df_check_kflag_member,
-	.log_details = btf_ref_type_log,
+	.log_details = btf_fwd_type_log,
 	.seq_show = btf_df_seq_show,
 };