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.fr
Signed-off-by: Jens Axboe <axboe@kernel.dk>