cgroup: Use RCU_INIT_POINTER(x, NULL) in cgroup.c

This patch replaces rcu_assign_pointer(x, NULL) with
RCU_INIT_POINTER(x, NULL)

The rcu_assign_pointer() ensures that the initialization of a
structure is carried out before storing a pointer to that structure.
And in the case of the NULL pointer, there is no structure to
initialize.  So, rcu_assign_pointer(p, NULL) can be safely converted
to RCU_INIT_POINTER(p, NULL)

Signed-off-by: Monam Agarwal <monamagarwal123@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cgroup.c

@@ -3545,7 +3545,7 @@ static void css_release(struct percpu_ref *ref)
 	struct cgroup_subsys_state *css =
 		container_of(ref, struct cgroup_subsys_state, refcnt);
 
-	rcu_assign_pointer(css->cgroup->subsys[css->ss->id], NULL);
+	RCU_INIT_POINTER(css->cgroup->subsys[css->ss->id], NULL);
 	call_rcu(&css->rcu_head, css_free_rcu_fn);
 }