Fix rate limiting

Commit e0224c6b accidentally broke rate limiting, since
it bypassed the sleep logic. Rework it slightly to not
skip that.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/io_u.c b/io_u.c
index 6598881..1013c7f 100644
--- a/io_u.c
+++ b/io_u.c
@@ -490,14 +490,14 @@
 
 	assert(ddir_rw(ddir));
 
-	if (td->rate_pending_usleep[ddir] <= 0 || !td->o.rwmix[odir])
+	if (td->rate_pending_usleep[ddir] <= 0)
 		return ddir;
 
 	/*
 	 * We have too much pending sleep in this direction. See if we
 	 * should switch.
 	 */
-	if (td_rw(td)) {
+	if (td_rw(td) && td->o.rwmix[odir]) {
 		/*
 		 * Other direction does not have too much pending, switch
 		 */