NetLabel: fixup the handling of CIPSOv4 tags to allow for multiple tag types

While the original CIPSOv4 code had provisions for multiple tag types the
implementation was not as great as it could be, pushing a lot of non-tag
specific processing into the tag specific code blocks.  This patch fixes that
issue making it easier to support multiple tag types in the future.

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