drivers/rnbd: restore sysfs interface to rnbd-client

Commit 137380c0 renamed 'rnbd-client' to 'rnbd_client', this changed
sysfs interface to /sys/devices/virtual/rnbd_client/ctl/map_device
from /sys/devices/virtual/rnbd-client/ctl/map_device.

CC: Ivan Orlov <ivan.orlov0322@gmail.com>
CC: "Md. Haris Iqbal" <haris.iqbal@ionos.com>
CC: Jack Wang <jinpu.wang@ionos.com>
Fixes: 137380c0 ("block/rnbd: make all 'class' structures const")
Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
Acked-by: Jack Wang <jinpu.wang@ionos.com>
Link: https://lore.kernel.org/r/20230816022210.2501228-1-lizhijian@fujitsu.com


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