NetLabel: check for a CIPSOv4 option before we do call into the CIPSOv4 layer

Right now the NetLabel code always jumps into the CIPSOv4 layer to determine if
a CIPSO IP option is present.  However, we can do this check directly in the
NetLabel code by making use of the CIPSO_V4_OPTEXIST() macro which should save
us a function call in the common case of not having a CIPSOv4 option present.

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