stat: update description of clat accounting in stat.h

stat: update description of clat accounting in stat.h

The description in stat.h of the data structure used for
storing completion latency percentiles had not been updated
to reflect the transition to nanosecond timing. This patch
updates the description to be consistent with the nanosecond
changes. There have been a few instances on github and the
mailing list where this had caused confusion.

Signed-off-by: Jens Axboe <axboe@kernel.dk>