rbd-nbd: fix unused nbd device search bug in container

rbd-nbd: fix unused nbd device search bug in container

In some container scenarios, the host may choose to
map a specific nbd device, for example, /dev/nbd6 into the
container, in that case, the nbd device available in the
container is not numbered from 0. The current unused
nbd device search function will return no result.
This patch fixes it.

Fixes: http://tracker.ceph.com/issues/22012
Signed-off-by: Li Wang <laurence.liwang@gmail.com>
Reviewed-by: Yunchuan Wen <yunchuan.wen@kylin-cloud.com>
(cherry picked from commit be0f9581f9727187ca03232e0b368e7da7a60609)