block/rnbd: Constify struct kobj_type

'struct kobj_type' is not modified in this driver. It is only used with
kobject_init_and_add() which takes a "const struct kobj_type *" parameter.

Constifying this structure moves some data to a read-only section, so
increase overall security.

On a x86_64, with allmodconfig, as an example:
Before:
======
   text	   data	    bss	    dec	    hex	filename
   4082	    792	      8	   4882	   1312	drivers/block/rnbd/rnbd-srv-sysfs.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
   4210	    672	      8	   4890	   131a	drivers/block/rnbd/rnbd-srv-sysfs.o
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Link: https://lore.kernel.org/r/e3d454173ffad30726c9351810d3aa7b75122711.1720462252.git.christophe.jaillet@wanadoo.frSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/rnbd/rnbd-clt-sysfs.c

@@ -475,7 +475,7 @@ void rnbd_clt_remove_dev_symlink(struct rnbd_clt_dev *dev)
 	}
 }
 
-static struct kobj_type rnbd_dev_ktype = {
+static const struct kobj_type rnbd_dev_ktype = {
 	.sysfs_ops      = &kobj_sysfs_ops,
 	.default_groups = rnbd_dev_groups,
 };

drivers/block/rnbd/rnbd-srv-sysfs.c

@@ -33,7 +33,7 @@ static void rnbd_srv_dev_release(struct kobject *kobj)
 	kfree(dev);
 }
 
-static struct kobj_type dev_ktype = {
+static const struct kobj_type dev_ktype = {
 	.sysfs_ops = &kobj_sysfs_ops,
 	.release = rnbd_srv_dev_release
 };
@@ -184,7 +184,7 @@ static void rnbd_srv_sess_dev_release(struct kobject *kobj)
 	rnbd_destroy_sess_dev(sess_dev, sess_dev->keep_id);
 }
 
-static struct kobj_type rnbd_srv_sess_dev_ktype = {
+static const struct kobj_type rnbd_srv_sess_dev_ktype = {
 	.sysfs_ops	= &kobj_sysfs_ops,
 	.release	= rnbd_srv_sess_dev_release,
 };