btrfs: remove orig_bytes from reserve_ticket

Now that we do not do partial filling of tickets simply remove
orig_bytes, it is no longer needed.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>