page allocator: slab: use nr_online_nodes to check for a NUMA platform

SLAB currently avoids checking a bitmap repeatedly by checking once and
storing a flag.  When the addition of nr_online_nodes as a cheaper version
of num_online_nodes(), this check can be replaced by nr_online_nodes.

(Christoph did a patch that this is lifted almost verbatim from)

Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Reviewed-by: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Dave Hansen <dave@linux.vnet.ibm.com>
Cc: Lee Schermerhorn <Lee.Schermerhorn@hp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/slab.c

@@ -898,7 +898,6 @@ static void __slab_error(const char *function, struct kmem_cache *cachep,
   */
 
 static int use_alien_caches __read_mostly = 1;
-static int numa_platform __read_mostly = 1;
 static int __init noaliencache_setup(char *s)
 {
 	use_alien_caches = 0;
@@ -1457,10 +1456,8 @@ void __init kmem_cache_init(void)
 	int order;
 	int node;
 
-	if (num_possible_nodes() == 1) {
+	if (num_possible_nodes() == 1)
 		use_alien_caches = 0;
-		numa_platform = 0;
-	}
 
 	for (i = 0; i < NUM_INIT_LISTS; i++) {
 		kmem_list3_init(&initkmem_list3[i]);
@@ -3590,7 +3587,7 @@ static inline void __cache_free(struct kmem_cache *cachep, void *objp)
 	 * variable to skip the call, which is mostly likely to be present in
 	 * the cache.
 	 */
-	if (numa_platform && cache_free_alien(cachep, objp))
+	if (nr_online_nodes > 1 && cache_free_alien(cachep, objp))
 		return;
 
 	if (likely(ac->avail < ac->limit)) {