blk-mq timeout handling fixes

[Christoph Hellwig]

This series fixes various issues with timeout handling that Robert
ran into when testing scsi-mq heavily.  He tested an earlier version,
and couldn't reproduce the issues anymore, although the series changed
quite significantly since and should probably be retested.

In summary we not only start the blk-mq timer inside the drivers
->queue_rq method after the request has been fully setup, and we
also tell the drivers if we're timing out a reserved (internal)
request or a real one.  Many drivers including will need to handle
those internal ones differently, e.g. for scsi-mq we don't even
have a scsi command structure allocated for the reserved commands.

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