mm/slab: Sharing s_next and s_stop between slab and slub

This patch shares s_next and s_stop between slab and slub.

Acked-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>