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>
diff --git a/libfio.c b/libfio.c
index ac629dc..433fee4 100644
--- a/libfio.c
+++ b/libfio.c
@@ -119,6 +119,7 @@
 		td->io_issues[i] = 0;
 		td->ts.total_io_u[i] = 0;
 		td->ts.runtime[i] = 0;
+		td->rwmix_issues = 0;
 	}
 
 	fio_gettime(&tv, NULL);