rnbd-clt: kill read_only from struct rnbd_clt_dev

The member is not needed since we can call get_disk_ro to achieve the
same goal.

Acked-by: Jack Wang <jinpu.wang@ionos.com>
Signed-off-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Link: https://lore.kernel.org/r/20220706133152.12058-4-guoqing.jiang@linux.dev


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