block: Fix default IO priority if there is no IO context

Upstream commit 53889bca ("block: make __get_task_ioprio() easier to
read") changes the IO priority returned to the caller if no IO context
is defined for the task. Prior to this commit, the returned IO priority
was determined by task_nice_ioclass() and task_nice_ioprio(). Now it is
always IOPRIO_DEFAULT, which translates to IOPRIO_CLASS_NONE with priority
0. However, task_nice_ioclass() returns IOPRIO_CLASS_IDLE, IOPRIO_CLASS_RT,
or IOPRIO_CLASS_BE depending on the task scheduling policy, and
task_nice_ioprio() returns a value determined by task_nice(). This causes
regressions in test code checking the IO priority and class of IO
operations on tasks with no IO context.

Fix the problem by returning the IO priority calculated from
task_nice_ioclass() and task_nice_ioprio() if no IO context is defined
to match earlier behavior.

Fixes: 53889bca ("block: make __get_task_ioprio() easier to read")
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/20250731044953.1852690-1-linux@roeck-us.net


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