manage nested subvolumes

In some case you may have sub volumes whose parent folder is not a btrfs (sub)volume, i.e.:
(assuming / is a btrfs volume)
# btrfs su li /mnt/
ID 293 gen 34693 top level 5 path mnt/subvol
ID 295 gen 34681 top level 293 path mnt/subvol/subvol2

 btrfs su li /mnt/subvol/
ID 293 gen 34693 top level 5 path mnt/subvol
ID 295 gen 34681 top level 293 path subvol2

The initial check would fail while trying to create /mnt/subvol.