Fix rate iops with ramp time

When the ramp time is over, fio resets the ->io_issues[]
counters, but not the rwmix counters. This causes a problem
for mixed read/write workloads that are rate limited and
include a ramp time.

Fix it by resetting rwmix_issues with the regular issues.

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