NVMe induced NULL deref in bt_iter()

[Sagi Grimberg]

Hi Ming,

> Yeah, the above change is correct, for any canceling requests in this
> way we should use blk_mq_quiesce_queue().

I still don't understand why should blk_mq_flush_busy_ctxs hit a NULL
deref if we don't touch the tagset...

Also, I'm wandering in what case we shouldn't use
blk_mq_quiesce_queue()? Maybe we should unexport blk_mq_stop_hw_queues()
and blk_mq_start_stopped_hw_queues() and use the quiesce/unquiesce
equivalent always?

The only fishy usage is in nvme_fc_start_fcp_op() where if submission
failed the code stop the hw queues and delays it, but I think it should
be handled differently..