slub: use __SetPageSlab function to set PG_slab flag

To set page-flag, using SetPageXXXX() and __SetPageXXXX() is more
understandable and maintainable. So change it.

Change-Id: I1851c6ed229cf2908671e851737a00d59e4ff902
Signed-off-by: Joonsoo Kim <js1304@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>