mm: slub: make kobj_type structure constant

Since commit ee6d3dd4 ("driver core: make kobj_type constant.")
the driver core allows the usage of const struct kobj_type.

Take advantage of this to constify the structure definition to prevent
modification at runtime.
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

mm/slub.c

@@ -6059,7 +6059,7 @@ static const struct sysfs_ops slab_sysfs_ops = {
 	.store = slab_attr_store,
 };
 
-static struct kobj_type slab_ktype = {
+static const struct kobj_type slab_ktype = {
 	.sysfs_ops = &slab_sysfs_ops,
 	.release = kmem_cache_release,
 };