vldb_check: remove unreferenced mh entries with -fix

vldb_check: remove unreferenced mh entries with -fix

When running vldb_check with -fix, clear any mh extent entries which
are set but are not referenced by an address entry in the
IpMappedAddr table.  These unreferenced entries already generated a
warning.  This commit adds the feature to clear the unreferenced mh
entries using vldb_check -fix option.

Reviewed-on: http://gerrit.openafs.org/7616
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit fdd3b0fb1f8e6948c651d7f1822d17a78668e5ff)

Change-Id: I79c79b3499fce448ebf662246de9f623b1565995
Reviewed-on: http://gerrit.openafs.org/8211
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>