disallow removing the first data pool
Our lookup-by-ino code requires that we be able to look for all existing inodes in a well-known location — the default data pool. If a user removes this pool, then we no longer have the backtraces we need.
A more sophisticated solution would involve copying the data from the "old default" pool to the "new default" pool, and that would be hard. That basically leaves us with not permitting users to delete the first pool.