blk-ioprio: don't set bio priority if not needed

We don't need to write to the bio if:

1) No ioprio value has ever been assigned to the blkcg
2) We wouldn't anyway, depending on bio and blkcg IO priority

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>