netlabel: Add network address selectors to the NetLabel/LSM domain mapping

This patch extends the NetLabel traffic labeling capabilities to individual
packets based not only on the LSM domain but the by the destination address
as well.  The changes here only affect the core NetLabel infrastructre,
changes to the NetLabel KAPI and individial protocol engines are also
required but are split out into a different patch to ease review.

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