blk-mq: don't clear flush_rq from tags->rqs[]

commit 364b6181

 ("blk-mq: clearing flush request reference in
tags->rqs[]") is added to clear the to-be-free flush request from
tags->rqs[] for avoiding use-after-free on the flush rq.

Yu Kuai reported that blk_mq_clear_flush_rq_mapping() slows down boot time
by ~8s because running scsi probe which may create and remove lots of
unpresent LUNs on megaraid-sas which uses BLK_MQ_F_TAG_HCTX_SHARED and
each request queue has lots of hw queues.

Improve the situation by not running blk_mq_clear_flush_rq_mapping if
disk isn't added when there can't be any flush request issued.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reported-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20220616014401.817001-4-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>