stat: disable per prio stats where not needed

stat: disable per prio stats where not needed

In order to avoid allocating a clat_prio_stat array for threadstats that we
know will never be able to contain more than a single priority, introduce a
new member disable_prio_stat in struct thread_stat.

The naming prefix is disable, since we want the default value to be 0
(enabled). This is because in default case, we do want sum_thread_stats()
to generate a per prio stat array. Only in the case where we know that we
don't want per priority stats to be generated, should this member be set
to 1.

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/20220203192814.18552-14-Niklas.Cassel@wdc.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>