Btrfs: use helper to cleanup tree roots

free_root_pointers() has been introduced to cleanup all of tree roots,
so just use it instead.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>