f2fs: compress: fix to update isize when overwriting compressed file

We missed to update isize of compressed file in write_end() with
below case:

cluster size is 16KB

- write 14KB data from offset 0
- overwrite 16KB data from offset 0

Fixes: 4c8ff709 ("f2fs: support data compression")
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>