So as pointed out in the last run (after changing my mind deeper into the series) I think we should instead use a BLK_MQ_REQ_PREEMPT flag.
The preempt only makes sense at the request level, not for file system requests. For the legacy case we can add blk_get_request_flags that takes the BLK_MQ_REQ_* flags, which is a much saner interface than the current blk_get_request anyway.
