slab: use well-defined macro, virt_to_slab()

This is trivial change, just use well-defined macro.
Acked-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Pekka Enberg <penberg@iki.fi>

mm/slab.c

@@ -2865,7 +2865,6 @@ static inline void verify_redzone_free(struct kmem_cache *cache, void *obj)
 static void *cache_free_debugcheck(struct kmem_cache *cachep, void *objp,
 				   unsigned long caller)
 {
-	struct page *page;
 	unsigned int objnr;
 	struct slab *slabp;
 
@@ -2873,9 +2872,7 @@ static void *cache_free_debugcheck(struct kmem_cache *cachep, void *objp,
 
 	objp -= obj_offset(cachep);
 	kfree_debugcheck(objp);
-	page = virt_to_head_page(objp);
-
-	slabp = page->slab_page;
+	slabp = virt_to_slab(objp);
 
 	if (cachep->flags & SLAB_RED_ZONE) {
 		verify_redzone_free(cachep, objp);
@@ -3087,7 +3084,7 @@ static void *cache_alloc_debugcheck_after(struct kmem_cache *cachep,
 		struct slab *slabp;
 		unsigned objnr;
 
-		slabp = virt_to_head_page(objp)->slab_page;
+		slabp = virt_to_slab(objp);
 		objnr = (unsigned)(objp - slabp->s_mem) / cachep->size;
 		slab_bufctl(slabp)[objnr] = BUFCTL_ACTIVE;
 	}