rbd: constify device_type structure

Declare device_type structure as const as it is only stored in the
type field of a device structure. This field is of type const, so add
const to the declaration of device_type structure.

File size before:
   text	   data	    bss	    dec	    hex	filename
  61546	  11610	    208	  73364	  11e94	drivers/block/rbd.o

File size after:
   text	   data	    bss	    dec	    hex	filename
  61610	  11578	    208	  73396	  11eb4	drivers/block/rbd.o

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

drivers/block/rbd.c

@@ -4630,7 +4630,7 @@ static const struct attribute_group *rbd_attr_groups[] = {
 
 static void rbd_dev_release(struct device *dev);
 
-static struct device_type rbd_device_type = {
+static const struct device_type rbd_device_type = {
 	.name		= "rbd",
 	.groups		= rbd_attr_groups,
 	.release	= rbd_dev_release,