btrfs: qgroup: allow to remove qgroup which has parent but no child.
When a qgroup has parents but no child, it should be removable in
Theory I think. But currently, we can not remove it when it has
either parent or child.
At this time, there is no subvol or qgroup depending on it.
Just a qgroup 2/0 is its parent, but 2/0 can work well without
1/0. So I think 1/0 should be removalbe. But:
# btrfs qgroup destroy 1/0 /mnt
ERROR: unable to destroy quota group: Device or resource busy
This patch remove the check of qgroup->parent in removing it,
then we can remove a qgroup when it has a parent.
Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com> Signed-off-by: Chris Mason <clm@fb.com>