f2fs: fix to avoid overflow when left shifting page offset

We use following method to calculate size with current page index:
size = index << PAGE_SHIFT
If type of index has only 32-bits size, left shifting will incur overflow,
which makes result incorrect.

So let's cast index with 64-bits type to avoid such issue.

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