libaio,io_uring: improve cmdprio_percentage option

libaio,io_uring: improve cmdprio_percentage option

The cmdprio_percentage option of the libaio and io_uring engines defines
a single percentage that applies to all IO operations, regardless of
their direction. This prevents defining different high priority IO
percentages for reads and writes operations. This differentiation can
however be useful in the case of a mixed read-write workload (rwmixread
and rwmixwrite options).

Change the option definition to allow specifying a comma separated list
of percentages, 2 at most, one for reads and one for writes. If only a
single percentage is defined, it applies to both reads and writes as
before. The cmdprio_percentage option becomes an array of DDIR_RWDIR_CNT
elements indexed with enum fio_ddir values. The last entry of the array
(for DDIR_TRIM) is always 0.

Also create a new cmdprio helper file, engines/cmdprio.h,
such that we can avoid code duplication between io_uring and libaio
io engines. This helper file will be extended in subsequent patches.

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>