mm/slub: use helper macro __ATTR_XX_MODE for SLAB_ATTR(_RO)

This allows more concise code, and VERIFY_OCTAL_PERMISSIONS() can help
validate any future change.

Signed-off-by: Lianjie Zhang <zhanglianjie@uniontech.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Link: https://lore.kernel.org/r/20220306073818.15089-1-zhanglianjie@uniontech.com