slab: introduce slab_max_order kernel parameter

Introduce new slab_max_order kernel parameter which is the equivalent of
slub_max_order.

For immediate purposes, allows users to override the heuristic that sets
the max order to 1 by default if they have more than 32MB of RAM.  This
may result in page allocation failures if there is substantial
fragmentation.

Another usecase would be to increase the max order for better
performance.

Acked-by: Christoph Lameter <cl@linux.com>
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>