Fix skewed latencies for rate IOPS

- when -rate_iops is specified, FIO periodically calls usleep() to limit IOPS

- Before usleep(), FIO always wait until the completion of all pending I/O

- For all I/O completions, FIO shows erroneous behavior of logging their
  latency, with that of the longest latency IO sample.

- w/ QD=8/ mixed R:W=33:66, up to 7 samples may get false latency log when
  waiting all I/O completions => False report of latency distribution

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