NetLabel: use the correct CIPSOv4 MLS label limits

The CIPSOv4 engine currently has MLS label limits which are slightly larger
than what the draft allows.  This is not a major problem due to the current
implementation but we should fix this so it doesn't bite us later.

Signed-off-by: Paul Moore <paul.moore@hp.com>
Signed-off-by: James Morris <jmorris@namei.org>