cgroup: fix RCU accesses around task->cgroups
cgroup: fix RCU accesses around task->cgroups

There are several places in kernel/cgroup.c where task->cgroups is
accessed and modified without going through proper RCU accessors.
None is broken as they're all lock protected accesses; however, this
still triggers sparse RCU address space warnings.

* Consistently use task_css_set() for task->cgroups dereferencing.

* Use RCU_INIT_POINTER() to clear task->cgroups to &init_css_set on
  exit.

* Remove unnecessary rcu_dereference_raw() from cset->subsys[]
  dereference in cgroup_exit().

Signed-off-by: Tejun Heo <546b05909706652891a87f7bfe385ae147f61f91@kernel.org>
Reported-by: Fengguang Wu <24f7fe9d205c8a9f6ade0c2894e14303ca16087f@intel.com>
Acked-by: Li Zefan <a8167b9d27aca9c6624754deb52c4c6420eded04@huawei.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs