writeback: fix uninitialized task_ratelimit

In balance_dirty_pages() task_ratelimit may be not initialized
(initialization skiped by goto pause), and then used when calling
tracing hook.

Fix it by moving the task_ratelimit assignment before goto pause.
Reported-by: NWitold Baryluk <baryluk@smp.if.uj.edu.pl>
Signed-off-by: NWu Fengguang <fengguang.wu@intel.com>

mm/page-writeback.c

@@ -1097,13 +1097,13 @@ static void balance_dirty_pages(struct address_space *mapping,
 		pos_ratio = bdi_position_ratio(bdi, dirty_thresh,
 					       background_thresh, nr_dirty,
 					       bdi_thresh, bdi_dirty);
-		if (unlikely(pos_ratio == 0)) {
+		task_ratelimit = ((u64)dirty_ratelimit * pos_ratio) >>
+							RATELIMIT_CALC_SHIFT;
+		if (unlikely(task_ratelimit == 0)) {
 			pause = max_pause;
 			goto pause;
 		}
-		task_ratelimit = (u64)dirty_ratelimit *
-					pos_ratio >> RATELIMIT_CALC_SHIFT;
-		pause = (HZ * pages_dirtied) / (task_ratelimit | 1);
+		pause = HZ * pages_dirtied / task_ratelimit;
 		if (unlikely(pause <= 0)) {
 			trace_balance_dirty_pages(bdi,
 						  dirty_thresh,