cpuset: fix a regression in validating config change

It's not allowed to clear masks of a cpuset if there're tasks in it,
but it's broken:

  # mkdir /cgroup/sub
  # echo 0 > /cgroup/sub/cpuset.cpus
  # echo 0 > /cgroup/sub/cpuset.mems
  # echo $$ > /cgroup/sub/tasks
  # echo > /cgroup/sub/cpuset.cpus
  (should fail)

This bug was introduced by commit 88fa523b
("cpuset: allow to move tasks to empty cpusets").

tj: Dropped temp bool variables and nestes the conditionals directly.

Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>