make salvager able to cope with corrupt class index

make salvager able to cope with corrupt class index

right now if a directory ends up in the small vnode index, the salvager
will never fix it. unlinking from the index (and recovering an orphan later)
beats crashing forever

Reviewed-on: http://gerrit.openafs.org/309
Reviewed-by: Russ Allbery <rra@stanford.edu>
Tested-by: Russ Allbery <rra@stanford.edu>