`qgroup destroy`: warn if qgroup cannot be removed after subvolume deletion

[kdave]

Removing a stale qgroup may not succeed for several reasons:

https://lore.kernel.org/linux-btrfs/fa8f65936f31c07ca4783229140ecfb1f7ada785.1714082499.git.wqu@suse.com/ :

• Currently kernel doesn not allow to delete non-empty qgroups
• A qgroup can only be empty after fully dropped and a transaction is committed
• Things like drop_subtree_threshold or other operations can mark qgroup inconsistent and skip accounting

The option --delete-qgroup has been removed but the same can happen with btrfs qgroup destroy as it's the same implementation by an ioctl.

We need to catch the conditions preventing the deletion and warn, or add options that will try to do the right thing at some cost, like waiting until the subvolume is cleaned.

[adam900710]

This sounds very reasonable and would be much more user friendly.

I'll add this feature soon.