Presence of backups breaks CSI snapshotting

[blampe]

Jotting down an issue to go along with #314 and piraeusdatastore/linstor-csi#174.

In summary, listBackups doesn't consistently respect the snap_name parameter. If any backups are present in the cluster, they are always returned by the endpoint even if they don't match the provided snap_name.

This completely breaks snapshotting via CSI. This endpoint is used to determine whether a snapshot ID is already in use, however because a value is always returned it appears to the CSI controller that every snapshot ID is in use. It will consistently fail to snapshot as long as any backups exists in the cluster.