os-linux: Cleanup IO priority class and value macros

os-linux: Cleanup IO priority class and value macros

In os/os-linux.h, define the ioprio() macro using the already defined
IOPRIO_MAX_PRIO macro instead of hard coding the maximum priority value
again. Also move the definitions of the ioprio_class() and ioprio()
macros before the ioprio_value() function and use ioprio_class() inside
ioprio_value_is_class_rt() instead of re-coding the iopriority class
extraction again in that function.

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Niklas Cassel <niklas.cassel@wdc.com>
Link: https://lore.kernel.org/r/20230721110510.44772-2-dlemoal@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>