cgroup: always use RCU accessors for protected accesses

kernel/cgroup.c still has places where a RCU pointer is set and
accessed directly without going through RCU_INIT_POINTER() or
rcu_dereference_protected().  They're all properly protected accesses
so nothing is broken but it leads to spurious sparse RCU address space
warnings.

Substitute direct accesses with RCU_INIT_POINTER() and
rcu_dereference_protected().  Note that %true is specified as the
extra condition for all derference updates.  This isn't ideal as all
it does is suppressing warning without actually policing
synchronization rules; however, most are scheduled to be removed
pretty soon along with css_id itself, so no reason to be more
elaborate.

Combined with the previous changes, this removes all RCU related
sparse warnings from cgroup.

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