cmdprio: add support for a new cmdprio_bssplit entry format

cmdprio: add support for a new cmdprio_bssplit entry format

Add support for a new cmdprio_bssplit format, while keeping support for the
old format, by migrating to the split_parse_prio_ddir() parsing function.

In this new format, a priority class and priority level is defined inside
each entry itself. In comparison with the old format, the new format does
not restrict all entries to share the same priority class and priority
level.

Therefore, this new format is very useful if you need to submit I/Os with
multiple IO priority class + IO priority level combinations, e.g. when
testing or verifying an IO scheduler.

cmdprio will allocate a clat_prio_stat array that holds all unique
priorities (including the default priority). Finally, it will set the
clat_prio pointer in the struct thread_stat (td->ts.clat_prio) to the
newly allocated array.

We also add a clat_prio_stat index to io_u.h, that will inform which array
element (which priority value) this specific I/O was submitted with.
The clat_prio_stat index will be used by the stat.c code, to avoid a costly
search operation to find the correct array element to use, for each and
every add_sample().

Note that while this patch will send down the correct I/O pattern to the
drive (potentially using multiple different priorities), it will not
display the cmdprio_{bssplit,percentage} stats correctly until a later
commit in the series (which changes stat.c to report clat stats on a per
priority granularity). This was done to ease reviewing.

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/20220203192814.18552-9-Niklas.Cassel@wdc.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>