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.

Change-Id: I941d714c825d7990083ecd15fd7bd9cd3b5917b3
Reviewed-on: http://gerrit.openafs.org/7616
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>