volser: Adjust TLV tag range

Tag values up to D_MAX cannot be used as TLV tags, as the parsing
logic assumes anything up to D_MAX is a control tag.  Adjust
MIN_TLV_TAG to 21 so that the TLV range sits just above D_MAX.

Change-Id: I8d55f536b82e209847400b5ac6689364abe44090
Reviewed-on: http://gerrit.openafs.org/9942
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>