slab: Common definition for kmem_cache_node

Put the definitions for the kmem_cache_node structures together so that
we have one structure. That will allow us to create more common fields in
the future which could yield more opportunities to share code.

Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>