libaio,io_uring: introduce cmdprio_class and cmdprio options

libaio,io_uring: introduce cmdprio_class and cmdprio options

When the cmdprio_percentage option is used, the specified percentage of
IO will be issued with the highest priority class IOPRIO_CLASS_RT. This
priority class maps to the ATA NCQ "high" priority level and allows
exercising a SATA device to measure its command latency characteristics
in the presence of low and high priority commands.

Beside ATA NCQ commands, Linux block IO schedulers also support IO
priorities and will behave differently in the presence of IOs with
different IO priority classes and values. However, cmdprio_percentage
does not allow specifying all possible priority classes and values.

To solve this, introduce libaio and io_uring engine specific options
cmdprio_class and cmdprio. These new options are the equivalent
of the prioclass and prio options and allow specifying the priority
class and priority value to use for asynchronous I/Os when the
cmdprio_percentage option is used. If not specified, the I/O priority
class defaults to IOPRIO_CLASS_RT and the I/O priority value to 0,
as before. Similarly to the cmdprio_percentage option, these options
can specify different values for read and write I/Os using a comma
separated list.

The manpage, HOWTO and fiograph configuration file are updated to
document these new options.

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>