Add support for the enumerated tag (tag type #2) to the CIPSOv4 protocol.

The enumerated tag allows for 15 categories to be specified in a CIPSO option,
where each category is an unsigned 16 bit field with a maximum value of 65534.

See Documentation/netlabel/draft-ietf-cipso-ipsecurity-01.txt for more details.
Signed-off-by: Paul Moore <paul.moore@hp.com>
Signed-off-by: James Morris <jmorris@namei.org>