slub: disallow changing cpu_partial from userspace for debug caches

For caches with debugging enabled, "slub: Switch per cpu partial page
support off for debugging" changes cpu_partial to 0.  It shouldn't be
tunable from userspace for such caches, otherwise the same accounting
issues arise during validation.

This patch disallows tuning /sys/kernel/slab/cache/cpu_partial to be non-
zero for caches with debugging enabled.
Acked-by: Christoph Lameter <cl@linux.com>
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>

mm/slub.c

@@ -4649,6 +4649,8 @@ static ssize_t cpu_partial_store(struct kmem_cache *s, const char *buf,
 	err = strict_strtoul(buf, 10, &objects);
 	if (err)
 		return err;
+	if (objects && kmem_cache_debug(s))
+		return -EINVAL;
 
 	s->cpu_partial = objects;
 	flush_all(s);