Allow changing FS_TREE (subvolid=5) UUID

[Forza-tng]

btrfstune can change the fs uuid, but it does not change the uuid of the root subvol (id=5). This can lead to problems with tools like btrbk if such a filesystem is cloned.

It would be good if there existed an option to change the root subvol/FS_TREE uuid.

digint/btrbk#641