mm/page-writeback.c: initialize m_dirty to avoid compile warning

When building kernel with gcc 5.2, the below warning is raised:

  mm/page-writeback.c: In function 'balance_dirty_pages.isra.10':
  mm/page-writeback.c:1545:17: warning: 'm_dirty' may be used uninitialized in this function [-Wmaybe-uninitialized]
     unsigned long m_dirty, m_thresh, m_bg_thresh;

The m_dirty{thresh, bg_thresh} are initialized in the block of "if
(mdtc)", so if mdts is null, they won't be initialized before being used.
Initialize m_dirty to zero, also initialize m_thresh and m_bg_thresh to
keep consistency.

They are used later by if condition: !mdtc || m_dirty <=
dirty_freerun_ceiling(m_thresh, m_bg_thresh)

If mdtc is null, dirty_freerun_ceiling will not be called at all, so the
initialization will not change any behavior other than just ceasing the
compile warning.

(akpm: the patch actually reduces .text size by ~20 bytes on gcc-4.x.y)

[akpm@linux-foundation.org: add comment]
Signed-off-by: Yang Shi <yang.shi@linaro.org>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>