Btrfs: fix a mismerge in btrfs_balance()

Raid56 merge (merge commit e942f883) had mistakenly removed a call to
__cancel_balance(), which resulted in balance not cleaning up after itself
after a successful finish.  (Cleanup includes switching the state, removing
the balance item and releasing mut_ex_op testnset lock.)  Bring it back.
Reported-by: NDavid Sterba <dsterba@suse.cz>
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>
Signed-off-by: NChris Mason <chris.mason@fusionio.com>

fs/btrfs/volumes.c

@@ -3235,6 +3235,11 @@ int btrfs_balance(struct btrfs_balance_control *bctl,
 		update_ioctl_balance_args(fs_info, 0, bargs);
 	}
 
+	if ((ret && ret != -ECANCELED && ret != -ENOSPC) ||
+	    balance_need_close(fs_info)) {
+		__cancel_balance(fs_info);
+	}
+
 	wake_up(&fs_info->balance_wait_q);
 
 	return ret;