RFC: don't allow any access to unexported mounts for NFSv4

[Rick Macklem]

> The workarounds you propose contradict everything people are used to.
> They are not easy or you need to care what you put in the parent
> directories of the one you want to export. It basically means that you
> can only use NFSv4 on newly setup systems, upgraded or migrated ones
> look out of the question (yes, I'm over-simplificating a bit).
> 
> I really hope someone can come up with a fix for this, else it would
> mean I would not use NFSv4 anywhere.
> 
Ok, can I assume that's a vote for "leave the hack in"?

Beyond that, all I can say is the NFSv4 model (and protocol) is very
different (one tree rooted at some point). Conversion may or may not be
worth the pain, in any individual case.

rick