stat: save the default ioprio in struct thread_stat

stat: save the default ioprio in struct thread_stat

To be able to report clat stats on a per priority granularity (instead of
only high/low priority), we need to be able to get the priority value that
was used for the stats in clat_stat.

When a thread is using a single priority (e.g. option prio/prioclass is
used (without any cmdprio options)), all the clat stats for this thread
will be stored in clat_stat. The problem with this is sum_thread_stats()
does not know the priority value that corresponds to the stats stored in
clat_stat.

Since we cannot access td->ioprio from sum_thread_stats(), simply mirror
td->ioprio inside struct thread_stat.

This way, sum_thread_stats() will be able to reuse the global clat stats
in clat_stat, without the need to duplicate the data for per priority
stats, in the case where there is only a single priority in use.

Server version is intentionally not incremented, as it will be incremented
in a later patch in the series. No need to bump it multiple times for the
same patch series.

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/20220203192814.18552-4-Niklas.Cassel@wdc.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>