This just moves the prep_to_mq calls up in preparation of further removal
of BLKPREP_* usage.
Reviewed-by: NJohannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: NBart Van Assche <bvanassche@acm.org>
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NJens Axboe <axboe@kernel.dk>