Add number_ios= option

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/io_u.c b/io_u.c
index a35aafd..c50a17d 100644
--- a/io_u.c
+++ b/io_u.c
@@ -1413,6 +1413,9 @@
 		add_bw_sample(td, idx, bytes, &icd->time);
 
 	add_iops_sample(td, idx, bytes, &icd->time);
+
+	if (td->o.number_ios && !--td->o.number_ios)
+		td->done = 1;
 }
 
 static long long usec_for_io(struct thread_data *td, enum fio_ddir ddir)