f2fs: fix to show correct discard_granularity in sysfs

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>

fs/f2fs/sysfs.c

@@ -170,6 +170,8 @@ static ssize_t f2fs_sbi_store(struct f2fs_attr *a,
 				dcc->pend_list_tag[i] &= (~P_ACTIVE);
 		}
 		mutex_unlock(&dcc->cmd_lock);
+
+		*ui = t;
 		return count;
 	}