rcu: use WRITE_ONCE() for ->next and ->pprev of hlist_nulls

In rculist_nulls.h we can still see ordinary assignments to ->pprev and
->next of hlist_nulls.

As noted in the two patches below:
commit efd04f8 ("rcu: Use WRITE_ONCE() for assignments to ->next for
rculist_nulls")
commit 860c880 ("rcu: Use WRITE_ONCE() for assignments to ->pprev for
hlist_nulls")

We should use WRITE_ONCE().

Signed-off-by: Xuanqiang Luo <luoxuanqiang@kylinos.cn>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>

Author: Xuanqiang Luo

include/linux/rculist_nulls.h

@@ -138,7 +138,7 @@ static inline void hlist_nulls_add_tail_rcu(struct hlist_nulls_node *n,
 
 	if (last) {
 		WRITE_ONCE(n->next, last->next);
-		n->pprev = &last->next;
+		WRITE_ONCE(n->pprev, &last->next);
 		rcu_assign_pointer(hlist_nulls_next_rcu(last), n);
 	} else {
 		hlist_nulls_add_head_rcu(n, h);
@@ -148,8 +148,8 @@ static inline void hlist_nulls_add_tail_rcu(struct hlist_nulls_node *n,
 /* after that hlist_nulls_del will work */
 static inline void hlist_nulls_add_fake(struct hlist_nulls_node *n)
 {
-	n->pprev = &n->next;
-	n->next = (struct hlist_nulls_node *)NULLS_MARKER(NULL);
+	WRITE_ONCE(n->pprev, &n->next);
+	WRITE_ONCE(n->next, (struct hlist_nulls_node *)NULLS_MARKER(NULL));
 }
 
 /**