mm/slab: move struct kmem_cache_cpu declaration to slub.c

Nothing outside SLUB itself accesses the struct kmem_cache_cpu fields so
it does not need to be declared in slub_def.h. This allows also to move
enum stat_item.

Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: David Rientjes <rientjes@google.com>
Tested-by: David Rientjes <rientjes@google.com>
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Tested-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>