Btrfs: do not overwrite error return value in the balance ioctl

If the call to btrfs_balance() failed we would overwrite the error
returned to user space with -EFAULT if the call to copy_to_user() failed
as well. Fix that by calling copy_to_user() only if btrfs_balance()
returned success or was canceled.
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ioctl.c

@@ -4781,7 +4781,7 @@ static long btrfs_ioctl_balance(struct file *file, void __user *arg)
 	ret = btrfs_balance(fs_info, bctl, bargs);
 	bctl = NULL;
 
-	if (arg) {
+	if ((ret == 0 || ret == -ECANCELED) && arg) {
 		if (copy_to_user(arg, bargs, sizeof(*bargs)))
 			ret = -EFAULT;
 	}