block: fold get_task_io_context into set_task_ioprio

Fold get_task_io_context into its only caller, and simplify the code
as no reference to the I/O context is required to just set the ioprio
field.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20211209063131.18537-9-hch@lst.de


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