When using Ceph as SDS, cinder can not delete the source volume backup snapshot

[Bui Doan Dang]

if you use Ceph as the SDS for Cinder. When we backup a volume, cinder will creates a source snapshot in source volume, to use import-diff and export-diff, it will exist until we remove the source volume or, the backup.

But it has a problem in this codes:

see: https://opendev.org/openstack/cinder/src/branch/master/cinder/backup/drivers/ceph.py#L604-L617

The logic of this codes show that cinder can find the source volume if only if the source volume in the same pool with the backup.

So if the pool backup is not the same with the pool volume. The source snapshot in the source volume will never be deleted until the source volume is deleted

Reproduce this bug:

- using Ceph as SDS and config backup not in the same pool with volume

- Backup and delete the backup and you can see the remain source snapshot in source volume

Example of a volume running daily backup for 4 months:

```
root@cephmon [2]# rbd du volumes_ssd/volume-feaa57e7-fe86-4f0e-86c3-7b31a6fa696c
NAME PROVISIONED USED
<email address hidden> 80 GiB 8.0 GiB
<email address hidden> 80 GiB 7.6 GiB
<email address hidden> 80 GiB 1.5 GiB
<email address hidden> 80 GiB 1.3 GiB
<email address hidden> 80 GiB 1.4 GiB
<email address hidden> 80 GiB 1.3 GiB
<email address hidden> 80 GiB 1.5 GiB
<email address hidden> 80 GiB 1.3 GiB
<email address hidden> 80 GiB 1.3 GiB
<email address hidden> 80 GiB 1.3 GiB
<email address hidden> 80 GiB 1.3 GiB
<email address hidden> 80 GiB 1.3 GiB
<email address hidden> 80 GiB 1.3 GiB
<email address hidden> 80 GiB 1.4 GiB
<email address hidden> 80 GiB 1.4 GiB
<email address hidden> 80 GiB 1.5 GiB
<email address hidden> 80 GiB 1.5 GiB
<email address hidden> 80 GiB 1.3 GiB
<email address hidden> 80 GiB 1.3 GiB
<email address hidden> 80 GiB 1.3 GiB
<email address hidden> 80 GiB 1.4 GiB
<email address hidden> 80 GiB 1.7 GiB
<email address hidden> 80 GiB 1.7 GiB
<email address hidden> 80 GiB 1.5 GiB
<email address hidden> 80 GiB 1.3 GiB
<email address hidden> 80 GiB 1.4 GiB
<email address hidden> 80 GiB 1.3 GiB
<email address hidden> 80 GiB 1.5 GiB
<email address hidden> 80 GiB 1.5 GiB
<email address hidden> 80 GiB 1.2 GiB
<email address hidden> 80 GiB 1.6 GiB
<email address hidden> 80 GiB 1.3 GiB
<email address hidden> 80 GiB 1.4 GiB
<email address hidden> 80 GiB 1.5 GiB
<email address hidden> 80 GiB 1.6 GiB
<email address hidden> 80 GiB 1.6 GiB
<email address hidden> 80 GiB 1.5 GiB
<email address hidden> 80 GiB 1.7 GiB
<email address hidden> 80 GiB 1.4 GiB
<email address hidden> 80 GiB 1.3 GiB
<email address hidden> 80 GiB 1.3 GiB
<email address hidden> 80 GiB 1.4 GiB
<email address hidden> 80 GiB 1.5 GiB
<email address hidden> 80 GiB 1.6 GiB
<email address hidden> 80 GiB 1.9 GiB
<email address hidden> 80 GiB 1.5 GiB
<email address hidden> 80 GiB 1.3 GiB
<email address hidden> 80 GiB 1.3 GiB
<email address hidden> 80 GiB 1.6 GiB
<email address hidden> 80 GiB 1.5 GiB
<email address hidden> 80 GiB 1.4 GiB
<email address hidden> 80 GiB 1.7 GiB
<email address hidden> 80 GiB 1.5 GiB
<email address hidden> 80 GiB 1.3 GiB
<email address hidden> 80 GiB 1.3 GiB
<email address hidden> 80 GiB 1.5 GiB
<email address hidden> 80 GiB 1.6 GiB
<email address hidden> 80 GiB 1.6 GiB
<email address hidden> 80 GiB 1.3 GiB
<email address hidden> 80 GiB 1.4 GiB
<email address hidden> 80 GiB 1.3 GiB
<email address hidden> 80 GiB 1.2 GiB
<email address hidden> 80 GiB 1.5 GiB
<email address hidden> 80 GiB 1.5 GiB
<email address hidden> 80 GiB 1.7 GiB
<email address hidden> 80 GiB 1.3 GiB
<email address hidden> 80 GiB 1.3 GiB
<email address hidden> 80 GiB 1.3 GiB
<email address hidden> 80 GiB 1.9 GiB
<email address hidden> 80 GiB 1.4 GiB
<email address hidden> 80 GiB 1.6 GiB
<email address hidden> 80 GiB 1.5 GiB
<email address hidden> 80 GiB 1.7 GiB
<email address hidden> 80 GiB 1.3 GiB
<email address hidden> 80 GiB 1.3 GiB
<email address hidden> 80 GiB 1.3 GiB
<email address hidden> 80 GiB 1.5 GiB
<email address hidden> 80 GiB 1.3 GiB
<email address hidden> 80 GiB 2.5 GiB
<email address hidden> 80 GiB 1.3 GiB
<email address hidden> 80 GiB 1.6 GiB
<email address hidden> 80 GiB 1.6 GiB
<email address hidden> 80 GiB 1.7 GiB
volume-feaa57e7-fe86-4f0e-86c3-7b31a6fa696c 80 GiB 1.4 GiB
<TOTAL> 80 GiB 263 GiB
```