mm/slab: Give s_next and s_stop slab-specific names

Give s_next and s_stop slab-specific names instead of exporting
"s_next" and "s_stop".

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