writeback: set max_pause to lowest value on zero bdi_dirty

Some trace shows lots of bdi_dirty=0 lines where it's actually some
small value if w/o the accounting errors in the per-cpu bdi stats.

In this case the max pause time should really be set to the smallest
(non-zero) value to avoid IO queue underrun and improve throughput.
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>

mm/page-writeback.c

@@ -989,8 +989,7 @@ static unsigned long bdi_max_pause(struct backing_dev_info *bdi,
 	 *
 	 * 8 serves as the safety ratio.
 	 */
-	if (bdi_dirty)
-		t = min(t, bdi_dirty * HZ / (8 * bw + 1));
+	t = min(t, bdi_dirty * HZ / (8 * bw + 1));
 
 	/*
 	 * The pause time will be settled within range (max_pause/4, max_pause).