blk-mq: avoid to synchronize rcu inside blk_cleanup_queue()

blk-mq: avoid to synchronize rcu inside blk_cleanup_queue()

SCSI probing may synchronously create and destroy a lot of request_queues
for non-existent devices. Any synchronize_rcu() in queue creation or
destroy path may introduce long latency during booting, see detailed
description in comment of blk_register_queue().

This patch removes one synchronize_rcu() inside blk_cleanup_queue()
for this case, commit c2856ae2f315d75(blk-mq: quiesce queue before freeing queue)
needs synchronize_rcu() for implementing blk_mq_quiesce_queue(), but
when queue isn't initialized, it isn't necessary to do that since
only pass-through requests are involved, no original issue in
scsi_execute() at all.

Without this patch and previous one, it may take more 20+ seconds for
virtio-scsi to complete disk probe. With the two patches, the time becomes
less than 100ms.

Fixes: c2856ae2f315d75 ("blk-mq: quiesce queue before freeing queue")
Reported-by: Andrew Jones <[email protected]>
Cc: Omar Sandoval <[email protected]>
Cc: Bart Van Assche <[email protected]>
Cc: [email protected]
Cc: "Martin K. Petersen" <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Tested-by: Andrew Jones <[email protected]>
Signed-off-by: Ming Lei <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>