Functions btrfs_uuid_scan_kthread() and btrfs_create_uuid_tree() are for
UUID tree rescan and creation, it's not suitable for volumes.[ch].

Move them to uuid-tree.[ch] instead.
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>