slab: replace smp_read_barrier_depends() with lockless_dereference()

Recently lockless_dereference() was added which can be used in place of
hard-coding smp_read_barrier_depends().  The following PATCH makes the
change.
Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Cc: "Paul E. McKenney" <paulmck@us.ibm.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/slab.h

@@ -209,15 +209,15 @@ cache_from_memcg_idx(struct kmem_cache *s, int idx)
 
 	rcu_read_lock();
 	params = rcu_dereference(s->memcg_params);
-	cachep = params->memcg_caches[idx];
-	rcu_read_unlock();
 
 	/*
 	 * Make sure we will access the up-to-date value. The code updating
 	 * memcg_caches issues a write barrier to match this (see
 	 * memcg_register_cache()).
 	 */
-	smp_read_barrier_depends();
+	cachep = lockless_dereference(params->memcg_caches[idx]);
+	rcu_read_unlock();
+
 	return cachep;
 }