f2fs: clean up {cic,dic}.ref handling

{cic,dic}.ref should be initialized to number of compressed pages,
let's initialize it directly rather than doing w/
f2fs_set_compressed_page().

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