f2fs: fix return value of releasepage for node and data

If the return value of releasepage is equal to zero, the page cannot be reclaimed.
Instead, we should return 1 in order to reclaim clean pages.

Reviewed-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>