cmdprio: Add support for per I/O priority hint

cmdprio: Add support for per I/O priority hint

Introduce the new option cmdprio_hint to allow specifying I/O priority
hints per IO with the io_uring and libaio IO engines. A third acceptable
format for the cmdprio_bssplit option is also introduced to allow
specifying an I/O hint in addition to a priority class and level.

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Niklas Cassel <niklas.cassel@wdc.com>
Link: https://lore.kernel.org/r/20230721110510.44772-6-dlemoal@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>