cmdprio: do not allocate memory for unused data direction

cmdprio: do not allocate memory for unused data direction

All cmdprio options only support data directions read and write.
However, each cmdprio option allocates memory for ddir trim as well,
even though nothing is ever written to this memory.

Change this so that we don't allocate memory for something which is
never used.

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/20211112095428.158300-4-Niklas.Cassel@wdc.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>