dm sysfs: 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>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

drivers/md/dm-sysfs.c

@@ -120,7 +120,7 @@ static const struct sysfs_ops dm_sysfs_ops = {
 	.store	= dm_attr_store,
 };
 
-static struct kobj_type dm_ktype = {
+static const struct kobj_type dm_ktype = {
 	.sysfs_ops	= &dm_sysfs_ops,
 	.default_groups	= dm_groups,
 	.release	= dm_kobject_release,