Btrfs filesystems may be slow when listing snapshots and calculating
space usage for each.
And that space information is not needed and never used in the scipt.
So, add --disable-ununsed-space to all calls to btrfs.
See: https://github.com/Antynea/grub-btrfs/issues/91
Co-authored-by: Peter G <nephros@pearl.crownest.nephros.org>
