stat: add helper for resetting the latency buckets

stat: add helper for resetting the latency buckets

Add a helper for resetting the latency buckets, and call it where
appropriate.

This makes the code easier to read, and puts the reset of the DDIR_SYNC
latency buckets together with the other statements for DDIR_SYNC.

A follow up patch will also make use of this new helper function.

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/20220203192814.18552-13-Niklas.Cassel@wdc.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>