volscan: hide -mask option

The -mask option is unneccessary and sets a bad precedent, so
deprecate and hide this option. The vnodes of interest can be found
can be found easily and much more flexibly with a simple command
pipeline.

Reviewed-on: http://gerrit.openafs.org/10901
Reviewed-by: D Brashear <shadow@your-file-system.com>
Tested-by: D Brashear <shadow@your-file-system.com>
(cherry picked from commit 8119636976de651ff0b8ccdca6a1a703643f7447)

Change-Id: I21519d7c0e9b3e3900cf3cd4c561d1a5aed1ef90
Reviewed-on: http://gerrit.openafs.org/11276
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>