Btrfs: fix lockdep warning on clone ioctl

I'm no lockdep expert, but this appears to make the lockdep warning go
away for the i_mutex locking in the clone ioctl.
Signed-off-by: Sage Weil <sage@newdream.net>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/ioctl.c

@@ -1492,11 +1492,11 @@ static noinline long btrfs_ioctl_clone(struct file *file, unsigned long srcfd,
 	path->reada = 2;
 
 	if (inode < src) {
-		mutex_lock(&inode->i_mutex);
-		mutex_lock(&src->i_mutex);
+		mutex_lock_nested(&inode->i_mutex, I_MUTEX_PARENT);
+		mutex_lock_nested(&src->i_mutex, I_MUTEX_CHILD);
 	} else {
-		mutex_lock(&src->i_mutex);
-		mutex_lock(&inode->i_mutex);
+		mutex_lock_nested(&src->i_mutex, I_MUTEX_PARENT);
+		mutex_lock_nested(&inode->i_mutex, I_MUTEX_CHILD);
 	}
 
 	/* determine range to clone */