Fix rate iops with ramp time

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>