f2fs: remove duplicated type casting

Since DUMMY_WRITTEN_PAGE and ATOMIC_WRITTEN_PAGE have already been
converted as unsigned long type, we don't need do type casting again.

Signed-off-by: Xiaojun Wang <wangxiaojun11@huawei.com>
Reported-by: Jack Qiu <jack.qiu@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>