Now that css_task_iter_start/next_end() supports blocking while
iterating, there's no reason to use css_scan_tasks() which is more
cumbersome to use and scheduled to be removed.

Convert all css_scan_tasks() usages in cpuset to
css_task_iter_start/next/end().  This simplifies the code by removing
heap allocation and callbacks.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>