btrfs: qgroup: do not treat "ret > 0" as error when deleting a qgroup

[BUG]
There is a bug report that when removing a child qgroup, if removing the
qgroup requires a rescan, it will delete the on-disk relation and qgroup
items but not the in-memory qgroup rb entry.

This makes all later qgroup rescan to fail.

[CAUSE]
The call chain looks like the following:

 btrfs_remove_qgroup()
 |- __del_qgroup_relation()
 |  |- quick_update_accounting()
 |     Which returns 1, showing this operation will require
 |     a qgroup rescan
 |- if (ret) goto out;
    Which skips the del_qgroup_rb() call, leaving the in-memory qgroup
    rb entry still intact.

Then later qgroup rescan/update will always fail and mark qgroup
inconsistent:

 btrfs_run_qgroups()
 |- update_qgroup_info_item()
 |  |- btrfs_search_slot()
 |     Which returns -ENOENT, as the qgroup info item is already
 |     deleted.
 |- qgroup_mark_inconsistent()
 |- return -ENOENT;

[FIX]
Do not treat "ret > 0" as an error.

Reported-by: c.f.a@posteo.de
Link: https://lore.kernel.org/linux-btrfs/14846642-85d3-4afb-ad99-1b74b22f5f90@posteo.de/
Fixes: 9c8b35b1ba ("btrfs: quota: Automatically update related qgroups or mark INCONSISTENT flags when assigning/deleting a qgroup relations.")
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/qgroup.c

@@ -1853,7 +1853,7 @@ int btrfs_remove_qgroup(struct btrfs_trans_handle *trans, u64 qgroupid)
 					struct btrfs_qgroup_list, next_group);
 		ret = __del_qgroup_relation(trans, qgroupid,
 					    list->group->qgroupid);
-		if (ret)
+		if (ret < 0)
 			goto out;
 	}