slub: Fix build error in kmem_cache_open() with !CONFIG_SLUB_DEBUG

This build bug:

 mm/slub.c: In function 'kmem_cache_open':
 mm/slub.c:2476: error: 'disable_higher_order_debug' undeclared (first use in this function)
 mm/slub.c:2476: error: (Each undeclared identifier is reported only once
 mm/slub.c:2476: error: for each function it appears in.)

Triggers because there's no !CONFIG_SLUB_DEBUG definition for
disable_higher_order_debug.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>