mm/page-writeback.c: use setup_deferrable_timer

Use setup_deferrable_timer() instead of init_timer_deferrable() to
simplify the code.

Link: http://lkml.kernel.org/r/e8e3d4280a34facbc007346f31df833cec28801e.1488070291.git.geliangtang@gmail.com
Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page-writeback.c

@@ -650,9 +650,8 @@ int wb_domain_init(struct wb_domain *dom, gfp_t gfp)
 
 	spin_lock_init(&dom->lock);
 
-	init_timer_deferrable(&dom->period_timer);
-	dom->period_timer.function = writeout_period;
-	dom->period_timer.data = (unsigned long)dom;
+	setup_deferrable_timer(&dom->period_timer, writeout_period,
+			       (unsigned long)dom);
 
 	dom->dirty_limit_tstamp = jiffies;