btrfs: rename btrfs_root::highest_objectid to free_objectid
This reflects the true purpose of the member as it's being used solely
in context where a new objectid is being allocated. Future changes will
also change the way it's being used to closely follow this semantics.
Reviewed-by: Josef Bacik <josef@toxicpanda.com> Signed-off-by: Nikolay Borisov <nborisov@suse.com> Reviewed-by: David Sterba <dsterba@suse.com> Signed-off-by: David Sterba <dsterba@suse.com>