mq-deadline: don't call req_get_ioprio from the I/O completion handler

req_get_ioprio looks at req->bio to find the I/O priority, which is not
set when completing bios that the driver fully iterated through.

Stash away the dd_per_prio in the elevator private data instead of looking
it up again to optimize the code a bit while fixing the regression from
removing the per-request ioprio value.

Fixes: 6975c1a4 ("block: remove the ioprio field from struct request")
Reported-by: Chris Bainbridge <chris.bainbridge@gmail.com>
Reported-by: Sam Protsenko <semen.protsenko@linaro.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Chris Bainbridge <chris.bainbridge@gmail.com>
Tested-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20241126102136.619067-1-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>