Tell loader to ignore newer features enabled on the root pool.

There are many new features in ZoF. Most, if not all, do not effect read only usage.
Encryption in particular is enabled at the pool level but used at the dataset level.
The loader obviously will not be able to boot if the boot dataset is encrypted, but
should not care if some other dataset in the root pool is encrypted.

Reviewed by: allanjude
MFC after: 1 week