cgroup: fix RCU related sparse warnings

kn->priv which is a void * is used as a RCU pointer by cgroup.  When
dereferencing it, it was passing kn->priv to rcu_derefreence() without
casting it into a RCU pointer triggering address space mismatch
warning from sparse.  Fix them.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Acked-by: Acked-by: Zefan Li <lizefan@huawei.com>

kernel/cgroup/cgroup-v1.c

@@ -694,7 +694,7 @@ int cgroupstats_build(struct cgroupstats *stats, struct dentry *dentry)
 	 * @kn->priv is RCU safe.  Let's do the RCU dancing.
 	 */
 	rcu_read_lock();
-	cgrp = rcu_dereference(kn->priv);
+	cgrp = rcu_dereference(*(void __rcu __force **)&kn->priv);
 	if (!cgrp || cgroup_is_dead(cgrp)) {
 		rcu_read_unlock();
 		mutex_unlock(&cgroup_mutex);

kernel/cgroup/cgroup.c

@@ -4932,7 +4932,7 @@ struct cgroup_subsys_state *css_tryget_online_from_dir(struct dentry *dentry,
 	 * have been or be removed at any point.  @kn->priv is RCU
 	 * protected for this access.  See css_release_work_fn() for details.
 	 */
-	cgrp = rcu_dereference(kn->priv);
+	cgrp = rcu_dereference(*(void __rcu __force **)&kn->priv);
 	if (cgrp)
 		css = cgroup_css(cgrp, ss);