slab: restrict the number of objects in a slab

To prepare to implement byte sized index for managing the freelist
of a slab, we should restrict the number of objects in a slab to be less
or equal to 256, since byte only represent 256 different values.
Setting the size of object to value equal or more than newly introduced
SLAB_OBJ_MIN_SIZE ensures that the number of objects in a slab is less or
equal to 256 for a slab with 1 page.

If page size is rather larger than 4096, above assumption would be wrong.
In this case, we would fall back on 2 bytes sized index.

If minimum size of kmalloc is less than 16, we use it as minimum object
size and give up this optimization.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>