NetLabel: use cipso_v4_doi_search() for local CIPSOv4 functions

The cipso_v4_doi_search() function behaves the same as cipso_v4_doi_getdef()
but is a local, static function so use it whenever possibile in the CIPSOv4
code base.

Signed-of-by: Paul Moore <paul.moore@hp.com>

Signed-off-by: James Morris <jmorris@namei.org>