volscan: option to ignore dir vnode magic

Option to ignore directory vnode magic when looking
up the path names. Print which vnode failed when
doing the checks.

Reviewed-on: http://gerrit.openafs.org/6979
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry-picked from commit 22fbed08a6f23dbbc05fd567e6e8e21e4d1a9709)

Change-Id: Ia9b61b8c0e812fbaa680f62b4b924d818135dc87
Reviewed-on: http://gerrit.openafs.org/11272
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>