rculist: Replace reference to atomic_ops.rst

The hlist_nulls_for_each_entry_rcu() docbook header references the
atomic_ops.rst file, which was removed in commit f0400a77 ("atomic:
Delete obsolete documentation").  This commit therefore substitutes a
section in memory-barriers.txt discussing the use of barrier() in loops.

Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

include/linux/rculist_nulls.h

@@ -161,7 +161,7 @@ static inline void hlist_nulls_add_fake(struct hlist_nulls_node *n)
  *
  * The barrier() is needed to make sure compiler doesn't cache first element [1],
  * as this loop can be restarted [2]
- * [1] Documentation/core-api/atomic_ops.rst around line 114
+ * [1] Documentation/memory-barriers.txt around line 1533
  * [2] Documentation/RCU/rculist_nulls.rst around line 146
  */
 #define hlist_nulls_for_each_entry_rcu(tpos, pos, head, member)			\