blk-mq: allow to defer ->queue_rq invocations to workqueue

[Christoph Hellwig]

Drivers that need to do synchronous, blocking operations to do I/O generally
want to defer all I/O to a drÑver-private workqueue.  Examples for that are
the loop driver, rbd, or ubi block driver, and probably lots more that haven't
been evaluated yet.

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