Fix multiple btrfs roots

[kimono-koans]

/dev/zd0 on /mnt type btrfs (rw,relatime,ssd,space_cache=v2,subvolid=5,subvol=/)
/dev/zd0 on /mnt/garden type btrfs (rw,relatime,ssd,space_cache=v2,subvolid=256,subvol=/garden)
/dev/zd16 on /media type btrfs (rw,relatime,ssd,space_cache=v2,subvolid=5,subvol=/)
/dev/zd16 on /media/@pot type btrfs (rw,relatime,ssd,space_cache=v2,subvolid=256,subvol=/@pot)

It's possible to have multiple btrfs roots. Here we verify that any root is the sub's underlying root.