FROMLIST: f2fs: use IPU for cold files

(url: https://patchwork.kernel.org/patch/9886419/

)

We expect cold files write data sequentially, but sometimes some of small data
can be updated, which incurs fragmentation.
Let's avoid that.

Change-Id: Ib4a8db92e05bc88b1c7809707078efd249421045
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>