block: move __get_task_ioprio() into header file

We call this once per IO, which can be millions of times per second.
Since nobody really uses io priorities, or at least it isn't very
common, this is all wasted time and can amount to as much as 3% of
the total kernel time.

Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>