[FIX] slab: Alien caches must not be initialized if the allocation of the alien cache failed

[Christopher Lameter <cl@linux.com>]

From: Christoph Lameter <cl@linux.com>

Callers of __alloc_alien() check for NULL.
We must do the same check in __alloc_alien() after the allocation of
the alien cache to avoid potential NULL pointer dereferences
should the  allocation fail.

Fixes: 49dfc304ba241b315068023962004542c5118103 ("slab: use the lock on alien_cache, instead of the lock on array_cache")
Fixes: c8522a3a5832b843570a3315674f5a3575958a5 ("Slab: introduce alloc_alien")
Signed-off-by: Christoph Lameter <cl@linux.com>

Index: linux/mm/slab.c
===================================================================
--- linux.orig/mm/slab.c
+++ linux/mm/slab.c
@@ -666,8 +666,10 @@ static struct alien_cache *__alloc_alien
 	struct alien_cache *alc = NULL;

 	alc = kmalloc_node(memsize, gfp, node);
-	init_arraycache(&alc->ac, entries, batch);
-	spin_lock_init(&alc->lock);
+	if (alc) {
+		init_arraycache(&alc->ac, entries, batch);
+		spin_lock_init(&alc->lock);
+	}
 	return alc;
 }