Btrfs: don't panic if we get an error while balancing V2

A user reported an error where if we try to balance an fs after a device has
been removed it will blow up.  This is because we get an EIO back and this is
where BUG_ON(ret) bites us in the ass.  To fix we just exit.  Thanks,
Reported-by: Anand Jain <Anand.Jain@oracle.com>
Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/volumes.c

@@ -2098,7 +2098,8 @@ int btrfs_balance(struct btrfs_root *dev_root)
 					   chunk_root->root_key.objectid,
 					   found_key.objectid,
 					   found_key.offset);
-		BUG_ON(ret && ret != -ENOSPC);
+		if (ret && ret != -ENOSPC)
+			goto error;
 		key.offset = found_key.offset - 1;
 	}
 	ret = 0;