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>