mm, slub: add CONFIG_SLUB_TINY

For tiny systems that have used SLOB until now, SLUB might be
impractical due to its higher memory usage. To help with that, introduce
an option CONFIG_SLUB_TINY that modifies SLUB to use less memory.
This is done by sacrificing scalability, security and debugging
features, therefore not recommended for any system with more than 16MB
RAM.

This commit introduces the option and uses it to set other related
options in a way that reduces memory usage.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Reviewed-by: Christoph Lameter <cl@linux.com>