libaio,io_uring: introduce cmdprio_bssplit

libaio,io_uring: introduce cmdprio_bssplit

The cmdprio_percentage, cmdprio_class and cmdprio options allow
specifying different values for read and write operations. This enables
various IO priority issuing patterns even uner a mixed read-write
workload but does not allow differentiation within read and write
I/O operation types with different sizes when the bssplit option is
used.

Introduce the cmdprio_bssplit option to complement the use of the
bssplit option.  This new option has the same format as the bssplit
option, but the percentage values indicate the percentage of I/O
operations with a particular block size that must be issued with the
priority class and value specified by cmdprio_class and cmdprio.

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>