mm/slab: Introduce kmem_buckets typedef

Encapsulate the concept of a single set of kmem_caches that are used
for the kmalloc size buckets. Redefine kmalloc_caches as an array
of these buckets (for the different global cache buckets).

Signed-off-by: Kees Cook <kees@kernel.org>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>