slab: replace free and inuse in struct slab with newly introduced active

Now, free in struct slab is same meaning as inuse.
So, remove both and replace them with active.

Acked-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Pekka Enberg <penberg@iki.fi>