f2fs: fix wrong calculation on f_files in statfs

In f2fs_statfs(), f_files should be the total number of available inodes
instead of the currently allocated inodes.
So, this patch should resolve the reported bug below.

Note that, showing 10% usage is not a bug, since f2fs reveals whole volume size
as much as possible and shows the space overhead as *used*.
This policy is fair enough with respect to other file systems.

<Reported Bug>
(loop0 is backed by 1GiB file)

$ mkfs.f2fs /dev/loop0

F2FS-tools: Ver: 1.1.0 (2012-12-11)
Info: sector size = 512
Info: total sectors = 2097152 (in 512bytes)
Info: zone aligned segment0 blkaddr: 512
Info: format successful

$ mount /dev/loop0 mnt/

$ df mnt/
Filesystem     1K-blocks  Used Available Use% Mounted on
/dev/loop0       1046528 98312    929784  10%
/home/zeta/linux-devel/mtd-bench/mnt

$ df mnt/ -i
Filesystem     Inodes   IUsed  IFree IUse% Mounted on
/dev/loop0       1 -465918 465919     - /home/zeta/linux-devel/mtd-bench/mnt

Notice IUsed is negative. Also, 10% usage on a fresh f2fs seems too
much to be correct.
Reported-and-Tested-by: Ezequiel Garcia <elezegarcia@gmail.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/super.c

@@ -148,8 +148,8 @@ static int f2fs_statfs(struct dentry *dentry, struct kstatfs *buf)
 	buf->f_bfree = buf->f_blocks - valid_user_blocks(sbi) - ovp_count;
 	buf->f_bavail = user_block_count - valid_user_blocks(sbi);
 
-	buf->f_files = valid_inode_count(sbi);
-	buf->f_ffree = sbi->total_node_count - valid_node_count(sbi);
+	buf->f_files = sbi->total_node_count;
+	buf->f_ffree = sbi->total_node_count - valid_inode_count(sbi);
 
 	buf->f_namelen = F2FS_MAX_NAME_LEN;
 	buf->f_fsid.val[0] = (u32)id;