[OpenAFS] Mount points for .backup volumes

[Nickolai Zeldovich]

> A quick check of my CVS repository shows that this behaviour has been
> present at least since AFS 3.3a.

But the return code of EvalMountPoint() wasn't being checked until
rev 1.5 of afs_vnop_lookup.c (in the OpenAFS tree), so it would
ignore ELOOP and return ENOENT from another part of the code.
E.g., on an older Transarc client:

department-of-alchemy~> cd .OldFiles
department-of-alchemy~/.OldFiles> ls .OldFiles
.OldFiles: No such file or directory

(I've found that some programs don't like getting ELOOP in response
to lstat().  In particular some versions of find seem to abort the
traversal upon receiving such an error.)

-- kolya