snap can be removed while it is been using by rbd-nbd device
3 - minor
Pull request ID:
I maped a snap to rbd-nbd device by cmd:
- rbd-nbd map rbd/test@snap
but if i remove the snap accidentally, then i found the cmd: 'rbd-nbd list-mapped' can show my map list
but the nbd device disappeared look by cmd lsblk
so i think before we map a snap to a nbd device, we should protect the snap.