prune: respect snapshot flock

A snapshot that's currently being read can still appear in the prune
list, but should not be removed.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>

src/api2/admin/datastore.rs

@@ -727,7 +727,7 @@ fn prune(
             }));
 
             if !(dry_run || keep) {
-                datastore.remove_backup_dir(&info.backup_dir, true)?;
+                datastore.remove_backup_dir(&info.backup_dir, false)?;
             }
         }