slab: Common name for the per node structures

Rename the structure used for the per node structures in slab
to have a name that expresses that fact.

Acked-by: Glauber Costa <glommer@parallels.com>
Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>