enable generic interface (/dev/ngX) for unknown command sets v2

Christoph Hellwig
2022-07-18T05:25:12Z

Hi all,

this is a version of the patch from Joel rebased on a bunch of cleanups
from me that I think make the layering a lot cleaner. Let me know what
everyone things.

Changes since v1:
- log consistent messages when only passthrough is supported for
namespaces