slab: Move kmalloc related function defs

Move these functions higher up in slab.h so that they are grouped with other
generic kmalloc related definitions.

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