How to delete btrfs subvolumes with <FS_TREE> in their name

I was able to delete these snapshots by first mounting the whole btrfs volume (not the @ subvolume) and then working from there:

# mount /dev/mapper/whatever /mnt -o subvol=/
# ls /mnt
@
@apt-snapshot-2013-04-17_21:44:30
...

So at this point, all subvolumes (including the funky apt-snapshot ones) are visible in /mnt, so we can delete them:

# btrfs subvol delete /mnt/@apt-snapshot-2013-04-17_21:44:30
# umount /mnt