f2fs: fix to show correct discard_granularity in sysfs

commit 80647e5f4c728ecea7d9190c6e7163755ff6835c upstream.

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