bundles/zfs: fix typo

bundles/zfs/files/zfs-auto-snapshot

@@ -36,7 +36,7 @@ for line in check_output(['zfs', 'list', '-H', '-o', 'name']).splitlines():
     line = line.decode('UTF-8')
 
     for prefix in metadata.get('snapshot_never', set()):
-        if dataset.startswith(prefix):
+        if line.startswith(prefix):
             break
     else:
         datasets.add(line)