btrfs-util: when snapshotting make sure we don't descent into subvolumes we just...

btrfs-util: when snapshotting make sure we don't descent into subvolumes we just created

We already had a safety check in place that we don't end up descending
to the original subvolume again, but we also should avoid descending in
the newly created one.

This is particularly important if we make a snapshot below its source,
like we do in "systemd-nspawn --ephemeral -D /".

Closes https://bugs.freedesktop.org/show_bug.cgi?id=90803