vol: Nuke parent vol special inodes

When we "nuke" a volume, we delete all inodes we can find that are for
the given volume id. This currently means that if we nuke an RW volume
id, we delete all of the inodes for file data for the entire volume
group (since they're all stored in the VG id), but we do not delete
the special inodes for any non-RW volumes in that volume group. Those
special inodes left behind are not very useful, since we just deleted
all of the actual file data.

Currently this means that on namei, it's impossible to nuke the
special inodes for non-RW volumes, since the namei nuke will only look
in the subdir for the given volume id. If you give it the RW volume
id, it won't delete the special inodes as menioned above; if you give
it the RO volume id, it will only look in the RO subdir, and won't
find the RO special inodes in the RW subdir.

If a volume group is damaged in such a way that the salvager cannot
fix it (due to a bug), this means that it is impossible to get rid of
that volume group completely from the partition on namei without
manually running "rm -rf" on the relevant AFSIDat directory. Normally
we have a failsafe of running 'vos zap -force', but that doesn't work
for non-RW special inodes, as mentioned above.

So, in order to allow this 'vos zap -force' failsafe to work in
hopefully all situations, also delete the special inodes for the
parent volume. Use similar logic as exists in the salvager's
OnlyOneVolume function.

Change-Id: Id29da48a548c70b2b9ada1dd09f41cb59452bd11
Reviewed-on: http://gerrit.openafs.org/10256
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>