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>