rnbd-clt: adjust the layout of struct rnbd_clt_dev

rnbd-clt: adjust the layout of struct rnbd_clt_dev

While at it, let re-arrange the struct to remove holes.

Before, pahole reports

	/* size: 232, cachelines: 4, members: 17 */
	/* sum members: 224, holes: 2, sum holes: 8 */
	/* last cacheline: 40 bytes */

After the change, the report changes to

	/* size: 224, cachelines: 4, members: 17 */
	/* last cacheline: 32 bytes */

Acked-by: Jack Wang <jinpu.wang@ionos.com>
Signed-off-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Link: https://lore.kernel.org/r/20220706133152.12058-6-guoqing.jiang@linux.dev
Signed-off-by: Jens Axboe <axboe@kernel.dk>