unable to restore partitionless btrfs.

[enihcam]

repro step:

1. mkfs.btrfs /dev/sda # a partitionless btrfs
2. mount /dev/sda and create subvolumes
3. umount and fsarchiver savefs ./data.fsa /dev/sda
4. mkfs.btrfs /dev/sda again
5. fsarchiver restfs ./data.fsa id=0,dest=/dev/sda
6. mount and you see only data from the first subvolume