NetLabel: the CIPSOv4 passthrough mapping does not pass categories correctly

The CIPSO passthrough mapping had a problem when sending categories which
would cause no or incorrect categories to be sent on the wire with a packet.
This patch fixes the problem which was a simple off-by-one bug.

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