Missing time store in rwmix switch

Caused missing display of either read or write direction for a mixed
workload.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/io_u.c b/io_u.c
index 57b991a..74c3295 100644
--- a/io_u.c
+++ b/io_u.c
@@ -319,8 +319,10 @@
 			max_bytes = td->this_io_bytes[ddir];
 			if (max_bytes >=
 			    (td->o.size * td->o.rwmix[ddir] / 100)) {
-				if (!td->rw_end_set[ddir])
+				if (!td->rw_end_set[ddir]) {
 					td->rw_end_set[ddir] = 1;
+					fio_gettime(&td->rw_end[ddir], NULL);
+				}
 
 				ddir ^= 1;
 			}