[OpenAFS] check in c (linux) whether a directory entry is a mount point for an AFS volume

[Ken Hornstein]

>I'm not sure that the application will have the ability to stat the
>mount point object.  The OpenAFS cache manager will always provide the
>details of the target volume root directory unless the target volume
>cannot be located or accessed.

I can only say that this technique has worked for several decades and
still seems to work today.

--Ken