Btrfs: fix endless loop in balancing block groups

Qgroup function may overwrite the saved error 'err' with 0
in case quota is not enabled, and this ends up with a
endless loop in balance because we keep going back to balance
the same block group.

It really should use 'ret' instead.
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/relocation.c

@@ -4200,9 +4200,11 @@ static noinline_for_stack int relocate_block_group(struct reloc_control *rc)
 		err = PTR_ERR(trans);
 		goto out_free;
 	}
-	err = qgroup_fix_relocated_data_extents(trans, rc);
-	if (err < 0) {
-		btrfs_abort_transaction(trans, err);
+	ret = qgroup_fix_relocated_data_extents(trans, rc);
+	if (ret < 0) {
+		btrfs_abort_transaction(trans, ret);
+		if (!err)
+			err = ret;
 		goto out_free;
 	}
 	btrfs_commit_transaction(trans, rc->extent_root);