slab: initialize unused alien cache entry as NULL at alloc_alien_cache().

Comparing with existing code, it's a simpler way to use kzalloc_node()
to ensure that each unused alien cache entry is NULL.

CC: Eric Dumazet <eric.dumazet@gmail.com>
Acked-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Christoph Lameter <cl@linux-foundation.org>
Acked-by: Matt Mackall <mpm@selenic.com>
Signed-off-by: Haicheng Li <haicheng.li@linux.intel.com>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>

mm/slab.c

@@ -983,13 +983,11 @@ static struct array_cache **alloc_alien_cache(int node, int limit, gfp_t gfp)
 
 	if (limit > 1)
 		limit = 12;
-	ac_ptr = kmalloc_node(memsize, gfp, node);
+	ac_ptr = kzalloc_node(memsize, gfp, node);
 	if (ac_ptr) {
 		for_each_node(i) {
-			if (i == node || !node_online(i)) {
-				ac_ptr[i] = NULL;
+			if (i == node || !node_online(i))
 				continue;
-			}
 			ac_ptr[i] = alloc_arraycache(node, limit, 0xbaadf00d, gfp);
 			if (!ac_ptr[i]) {
 				for (i--; i >= 0; i--)