blk-mq: move queue_is_mq out of blk_mq_cancel_work_sync

The only caller that needs queue_is_mq check is del_gendisk, so move the
check into it.

Signed-off-by: Jinlong Chen <nickyc975@zju.edu.cn>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20221030094730.1275463-1-nickyc975@zju.edu.cn


Signed-off-by: Jens Axboe <axboe@kernel.dk>