cmdprio: add mode to make the logic easier to reason about

cmdprio: add mode to make the logic easier to reason about

Add a new field "mode", in order to know if we are determining IO
priorities according to cmdprio_percentage or to cmdprio_bssplit.

This makes the logic easier to reason about, and allows us to
remove the "use_cmdprio" variable from the ioengines themselves.

Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Link: https://lore.kernel.org/r/20211112095428.158300-8-Niklas.Cassel@wdc.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>