Currently `btrfs qgroup show` command shows any 0 level qgroup without a
root backref as `<stale>`, which is not correct.
There are several more cases:
- Under deletion
The subvolume is not yet full dropped, but unlinked.
In that case we would not have a root backref item, but the qgroup is
not stale.
- Squota space holder
This is for squota mode, that a fully dropped subvolume still have
extents accounting on the already-gone subvolume.
In this case it's not stale either, and future accounting relies on
it.
This patch would add above special cases, and add an extra `SPECIAL
PATHS` section to explain all the cases, including `<stale>`.
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
7f3ab46400