slab: use struct page for slab management

Now, there are a few field in struct slab, so we can overload these
over struct page. This will save some memory and reduce cache footprint.

After this change, slabp_cache and slab_size no longer related to
a struct slab, so rename them as freelist_cache and freelist_size.

These changes are just mechanical ones and there is no functional change.

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>

include/linux/slab_def.h

@@ -41,8 +41,8 @@ struct kmem_cache {
 
 	size_t colour;			/* cache colouring range */
 	unsigned int colour_off;	/* colour offset */
-	struct kmem_cache *slabp_cache;
-	unsigned int slab_size;
+	struct kmem_cache *freelist_cache;
+	unsigned int freelist_size;
 
 	/* constructor func */
 	void (*ctor)(void *obj);