Create backups of "/@"

First of all, thank you for this great script!
Unfortunately I cannot get it to work in my environment, maybe somebody can help. I'm sure I missed something, but I don't know what.

Current output of `btrfs subvolume list /`
```
ID 1419 gen 6620027 top level 5 path timeshift-btrfs/snapshots/2019-12-05_02-37-02/@
ID 1422 gen 6620027 top level 5 path timeshift-btrfs/snapshots/2019-12-07_18-48-50/@
ID 1428 gen 6621866 top level 5 path timeshift-btrfs/snapshots/2019-12-12_03-00-02/@
ID 1431 gen 6627067 top level 5 path @
ID 1432 gen 6621866 top level 5 path timeshift-btrfs/snapshots/2019-12-14_20-00-02/@
```
As you see, I am using timeshift for snapshots. I like to use it in the future too, because it has an GUI and is really easy to restore snapshots with it.

My root subvolume is `@` mounted at `/`. I read about converting the root subvolume `@` to a subvolume `rootfs`, but I thought it should work this way too...

Current output, when running btrbk dryrun:
```
btrbk command line client, version 0.29.0 (Sun Dec 15 13:07:03 2019)
Using configuration: /etc/btrbk/system-backup.conf
WARNING: Skipping subvolume "//@": Failed to fetch subvolume detail
WARNING: ... sh: readlink -v -e //@
WARNING: ... readlink: //@: No such file or directory
Completed within: 0s (Sun Dec 15 13:07:03 2019)
--------------------------------------------------------------------------------
Backup Summary (btrbk command line client, version 0.29.0)

Date: Sun Dec 15 13:07:03 2019
Config: /etc/btrbk/system-backup.conf
Dryrun: YES

Legend:
=== up-to-date subvolume (source snapshot)
+++ created subvolume (source snapshot)
--- deleted subvolume
*** received subvolume (non-incremental)
>>> received subvolume (incremental)
--------------------------------------------------------------------------------
//@
!!! Aborted: Failed to fetch subvolume detail

NOTE: Some errors occurred, which may result in missing backups!
Please check warning and error messages above.

NOTE: Dryrun was active, none of the operations above were actually executed!
```

Current `backup-system.conf`:
```
snapshot_dir btrbk_snapshot

snapshot_preserve_min 2d
snapshot_preserve 14d

target_preserve_min no
target_preserve 20d 10w *m

archive_preserve_min latest
archive_preserve 12m 10y

volume /
# no action if external disk is not attached
snapshot_create ondemand

# propagates to all subvolume sections:
target /mnt/347377a5-71a6-4803-a485-d0077ab1bc34/backup-system

subvolume @
```

Thank you!