stat: rename add_lat_percentile_sample()

stat: rename add_lat_percentile_sample()

The name for add_lat_percentile_sample() is confusing, since the function
actually adds a per priority percentile sample (it also adds a regular
sample), yet it doesn't have prio as part of the function name.

Rename the function so that it is more obvious that this function should
be used if you want to add a prio percentile sample.

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/20211125132020.109955-4-Niklas.Cassel@wdc.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>