blk-wbt: Optimize wbt_done() for non-throttled writes

blk-wbt: Optimize wbt_done() for non-throttled writes

In the current implementation, the sync_cookie and last_cookie members of
struct rq_wb are used only by read requests and not by non-throttled write
requests. Based on this, we can optimize wbt_done() by removing one if
condition check for non-throttled write requests.

Signed-off-by: Tang Yizhou <yizhou.tang@shopee.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20250727173959.160835-2-yizhou.tang@shopee.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>