writeback: do strict bdi dirty_exceeded

This helps to reduce dirty throttling polls and hence CPU overheads.

bdi->dirty_exceeded typically only helps when suddenly starting 100+
dd's on a disk, in which case the dd's may need to poll
balance_dirty_pages() earlier than tsk->nr_dirtied_pause.

CC: Jan Kara <jack@suse.cz>
CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>

mm/page-writeback.c

@@ -1160,7 +1160,7 @@ static void balance_dirty_pages(struct address_space *mapping,
 				    bdi_stat(bdi, BDI_WRITEBACK);
 		}
 
-		dirty_exceeded = (bdi_dirty > bdi_thresh) ||
+		dirty_exceeded = (bdi_dirty > bdi_thresh) &&
 				  (nr_dirty > dirty_thresh);
 		if (dirty_exceeded && !bdi->dirty_exceeded)
 			bdi->dirty_exceeded = 1;