It uses a single label and checks the validity of each pointer. This
is err-prone, and actually we had a bug because one of the check was
insufficient.

Use multi lables as we do in other places.

v2:
- drop initializations of local variables.
Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>