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>