[BTRFS] Show the `unallocated space` insted of `df`
On a BTRFS FS it should use the unallocated space and not the one reported by df.
Otherwise your device suddenly turns into the readonly mode until you balance the FS well enough.
On a BTRFS FS it should use the unallocated space and not the one reported by df.
Otherwise your device suddenly turns into the readonly mode until you balance the FS well enough.