Fix below incorrect display when reading discard_granularity sysfs node.

$ cat /sys/fs/f2fs/<device>/discard_granularity
$ 16
$ echo 32 > /sys/fs/f2fs/<device>/discard_granularity
$ cat /sys/fs/f2fs/<device>/discard_granularity
$ 16
Signed-off-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>