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

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 efd04f8a8b45 ("rcu: Use WRITE_ONCE() for assignments to ->next for
rculist_nulls")
commit 860c8802ace1 ("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>