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.

Change-Id: Ibe75928c6b041d135c0cb5867228947cd7f4e889
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>