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_RE
This patch does not change any functionality but makes the
REQ_PREEMPT flag available to blk_get_request(). A later patch
will add code to blk_get_request() that checks the REQ_PREEMPT
flag. Note: the IDE sense_rq request is allocated statically so
there is no blk_get_request() call that correspond
