fio: Introduce the log_prio option

fio: Introduce the log_prio option

Introduce the log_prio option to expand priority logging from just a
single bit information (priority high vs low) to the full value of the
priority value used to execute IOs. When this option is set, the
priority value is printed as a 16-bits hexadecimal value combining
the I/O priority class and priority level as defined by the
ioprio_value() helper.

Similarly to the log_offset option, this option does not result in
actual I/O priority logging when log_avg_msec is set.

This patch also fixes a problem with the IO_U_F_PRIORITY flag, namely
that this flag is used to indicate that the IO is being executed with a
high priority on the device while at the same time indicating how to
account for the IO completion latency (high_prio clat vs low_prio clat).
With the introduction of the cmdprio_class and cmdprio options, these
assumptions are not necesarilly compatible anymore.

These problems are addressed as follows:
* The priority_bit field of struct iosample is replaced with the
  16-bits priority field representing the full io_u->ioprio value. When
  log_prio is set, the priority field value is logged as is. When
  log_prio is not set, 1 is logged as the entry's priority field if the
  sample priority class is IOPRIO_CLASS_RT, and 0 otherwise.
* IO_U_F_PRIORITY is renamed to IO_U_F_HIGH_PRIO to indicate that a job
  IO has the highest priority within the job context and so must be
  accounted as such using high_prio clat.

While fio final statistics only show accounting of high vs low IO
completion latency statistics, the log_prio option allows a user to
perform more detailed statistical analysis of a workload using
multiple different IO priorities.

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>