f2fs: initialize page->private when using for our internal use

We need to guarantee it's initially zero. Otherwise, it'll hurt entire flag
operations.

Fixes: b763f3be ("f2fs: restructure f2fs page.private layout")
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>