btrfs: fix perms on demonstration debugfs interface

btrfs provides a helpful demonstration of how to export
a global variable via debugfs; however, it is unique among
other debugfs files in that it is world-writable, which causes
some concern to people who are not familiar with its purpose.

Fix it so that it is only user-writable.
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/sysfs.c

@@ -836,7 +836,7 @@ static int btrfs_init_debugfs(void)
 	if (!btrfs_debugfs_root_dentry)
 		return -ENOMEM;
 
-	debugfs_create_u64("test", S_IRUGO | S_IWUGO, btrfs_debugfs_root_dentry,
+	debugfs_create_u64("test", S_IRUGO | S_IWUSR, btrfs_debugfs_root_dentry,
 			&btrfs_debugfs_test);
 #endif
 	return 0;