cgroup: Use cgroup_attach_{lock,unlock}() from cgroup_attach_task_all()

No behavior changes; preparing for potential locking changes in future.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reviewed-by: Mukesh Ojha <quic_mojha@quicinc.com>
Signed-off-by: Tejun Heo <tj@kernel.org>