btrfs: Allow unprivileged user to delete subvolumes (kernel >= 4.18)

Fix issue 41762

Cherry-pick "drivers: btrfs: Allow unprivileged user to delete subvolumes" from containers/storage
831e32b6bd

> In btrfs, subvolume can be deleted by IOC_SNAP_DESTROY ioctl but there
> is one catch: unprivileged IOC_SNAP_DESTROY call is restricted by default.
>
> This is because IOC_SNAP_DESTROY only performs permission checks on
> the top directory(subvolume) and unprivileged user might delete dirs/files
> which cannot be deleted otherwise. This restriction can be relaxed if
> user_subvol_rm_allowed mount option is used.
>
> Although the above ioctl had been the only way to delete a subvolume,
> btrfs now allows deletion of subvolume just like regular directory
> (i.e. rmdir sycall) since kernel 4.18.
>
> So if we fail to cleanup subvolume in subvolDelete(), just fallback to
> system.EnsureRmoveall() to try to cleanup subvolumes again.
> (Note: quota needs privilege, so if quota is enabled we do not fallback)
>
> This fix will allow non-privileged container works with btrfs backend.

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

daemon/graphdriver/btrfs/btrfs.go

@@ -633,7 +633,14 @@ func (d *Driver) Remove(id string) error {
 	d.updateQuotaStatus()
 
 	if err := subvolDelete(d.subvolumesDir(), id, d.quotaEnabled); err != nil {
-		return err
+		if d.quotaEnabled {
+			return err
+		}
+		// If quota is not enabled, fallback to rmdir syscall to delete subvolumes.
+		// This would allow unprivileged user to delete their owned subvolumes
+		// in kernel >= 4.18 without user_subvol_rm_allowed mount option.
+		//
+		// From https://github.com/containers/storage/pull/508/commits/831e32b6bdcb530acc4c1cb9059d3c6dba14208c
 	}
 	if err := system.EnsureRemoveAll(dir); err != nil {
 		return err