Btrfs: do not overwrite error return value in the device replace ioctl

If the call to btrfs_dev_replace_by_ioctl() 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 no error
happened before or a device replace operation 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

@@ -4475,7 +4475,7 @@ static long btrfs_ioctl_dev_replace(struct btrfs_fs_info *fs_info,
 		break;
 	}
 
-	if (copy_to_user(arg, p, sizeof(*p)))
+	if ((ret == 0 || ret == -ECANCELED) && copy_to_user(arg, p, sizeof(*p)))
 		ret = -EFAULT;
 out:
 	kfree(p);