fixed btrfs subvolumes without __active prefix

There are some subvolumes prefixed with __active and some not. This are fixes to handle both of them.
No, it's varying output from the `btrfs subvolume list` command. Looks like this (with or without `__active/`:

`ID 74980 gen 650760 top level 260 path __active/var/lib/conair/layers/48b2eb1cccc4da361a50065819ca364e265bc8ce`