mm: prevent to write out dirty page in CMA by may_writepage

Now, local variable references in shrink_page_list is
PAGEREF_RECLAIM_CLEAN as default. It is for preventing to reclaim
dirty pages when CMA try to migrate pages.
Strictly speaking, we don't need it because CMA already didn't allow
to write out by .may_writepage = 0 in reclaim_clean_pages_from_list.

Morever, it has a problem to prevent anonymous pages's swap out when
we use force_reclaim = true in shrink_page_list(ex, per process reclaim
can do it)

So this patch makes references's default value to PAGEREF_RECLAIM
and declare .may_writepage = 0 of scan_control in CMA part to make
code more clear.

Change-Id: I5edc3c955d106ecebc4949ce27daf5b7b7a18089
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Mel Gorman <mgorman@suse.de>
Reported-by: Minkyung Kim <minkyung88@lge.com>
Signed-off-by: Minchan Kim <minchan@kernel.org>
Patch-mainline: linux-mm @ 9 May 2013 16:21:23
[vinmenon@codeaurora.org: trivial merge conflict fixes]
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>