os-linux: add initial support for IO priority hints

os-linux: add initial support for IO priority hints

Add initial support for Linux to allow specifying a hint for any
priority value. With this change, a priority value becomes the
combination of a priority class, a priority level and a hint.
The generic os.h ioprio manipulation macros, as well as the
os-dragonfly.h ioprio manipulation macros are modified to ignore this
hint.

For all other OSes that do not support priority classes, priotity hints
are ignored and always equal to 0.

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Niklas Cassel <niklas.cassel@wdc.com>
Link: https://lore.kernel.org/r/20230721110510.44772-4-dlemoal@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>