block/rnbd-srv: rename one member in rnbd_srv_dev

It actually represents the name of rnbd_srv_dev.

Acked-by: Jack Wang <jinpu.wang@ionos.com>
Signed-off-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Link: https://lore.kernel.org/r/20230524070026.2932-6-guoqing.jiang@linux.dev


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