stat: add a new function to allocate a clat_prio_stat array

stat: add a new function to allocate a clat_prio_stat array

To be able to report clat stats on a per priority granularity (instead of
only high/low priority), we need a new function which allocates a
clat_prio_stat array. This array will hold the clat stats for all the
different priorities that will be used by the struct td.

The clat_prio_stat array will eventually replace io_u_plat_high_prio,
io_u_plat_low_prio, clat_high_prio_stat, and clat_low_prio_stat.

A follow up patch will convert stat.c to use the new array.

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/20220203192814.18552-6-Niklas.Cassel@wdc.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>