cpuset: use rcu_read_lock() to protect task_cs()

We no longer use task_lock() to protect tsk->cgroups.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>