t/io_uring: don't track IO latencies the first second of runtime

t/io_uring: don't track IO latencies the first second of runtime

The most variation is usually seen at startup, so don't start tracking
latencies until we've done the first reporting run. Things should be
nice and stable at that point.

To make this cheaper on the fast path, clock_index is only valid if
it's non-zero. This makes checking for stats cheap in the reap path.

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