blk-mq: avoid double ->queue_rq() because of early timeout

In the Linux kernel, the following vulnerability has been resolved:

blk-mq: avoid double ->queue_rq() because of early timeout

David Jeffery found one double ->queue_rq() issue, so far it can

be triggered in VM use case because of long vmexit latency or preempt

latency of vCPU pthread or long page fault in vCPU pthread, then block

IO req could be timed out before queuing the request to hardware but after

calling blk_mq_start_request() during ->queue_rq(), then timeout handler

may handle it by requeue, then double ->queue_rq() is caused, and kernel

panic.

So far, it is driver's responsibility to cover the race between timeout

and completion, so it seems supposed to be solved in driver in theory,

given driver has enough knowledge.

But it is really one common problem, lots of driver could have similar

issue, and could be hard to fix all affected drivers, even it isn't easy

for driver to handle the race. So David suggests this patch by draining

in-progress ->queue_rq() for solving this issue.