os: introduce ioprio_value() helper

os: introduce ioprio_value() helper

Introduce the ioprio_value() helper function to calculate a priority
value based on a priority class and priority level. For Linux and
Android, this is defined as an integer equal to the priority class
shifted left by 13 bits and or-ed with the priority level. For
Dragonfly, ioprio_value() simply returns the priority level as there
is no concept of priority class.

Use this new helper in the io_uring and libaio engines to set IO
priority when the cmdprio_percentage option is used.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>