slob: Rework #ifdeffery in slab.h

Make the SLOB specific stuff harmonize more with the way the other allocators
do it. Create the typical kmalloc constants for that purpose. SLOB does not
support it but the constants help us avoid #ifdefs.

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