cgroup: drop task_lock() protection around task->cgroups

For optimization, task_lock() is additionally used to protect
task->cgroups.  The optimization is pretty dubious as either
css_set_rwsem is grabbed anyway or PF_EXITING already protects
task->cgroups.  It adds only overhead and confusion at this point.
Let's drop task_[un]lock() and update comments accordingly.

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