unprivileged `btrfs subvol list`

[maharmstone]

Resubmission of Omar's patches from June, which allow btrfs subvol list to work without CAP_SYS_ADMIN: https://lore.kernel.org/linux-btrfs/[email protected]/

@kdave, I appreciate that you were working on a replacement for btrfs subvol list in 2022, but we need this for unprivileged containers. Plus in any event we'll be keeping the old interface for a good while yet, in the interests of stability.

[kdave]

Well, not just in 2022, I've been working on that all the time (irregularly). I understand you need it but it's an interface change and currently there's the mkfs set of changes and subvol create/delete pots to libbtrfsutil with yet unresolved issues, so subvol list will wait.