[OpenAFS] Mount points for .backup volumes

[Nickolai Zeldovich]

> Actually, ELOOP is returned when the number of symbolic links followed is
> greater than the system defined limit, meaning the client was in fact
> traversing the mount points recursively.

I think this behavior might've changed recently; EvalMountPoint() in
afs/VNOPS/afs_vnop_lookup.c does infact return ELOOP when it sees a
backup volume mountpoint in a backup volume (but recursive traversal
of non-backup volumes is actually ok):

pepsi-one~> cd .OldFiles
pepsi-one~/.OldFiles> ls .OldFiles
.OldFiles: Number of symbolic links encountered during path name traversal exceeds MAXSYMLINKS

(One might argue, though, that this is the wrong error code to return
here.. Hmm.)

-- kolya