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>