slab: factor out calculate nr objects in cache_estimate

This logic is not simple to understand so that making separate function
helping readability. Additionally, we can use this change in the
following patch which implement for freelist to have another sized index
in according to nr objects.

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