NetLabel: correctly fill in unused CIPSOv4 level and category mappings

Back when the original NetLabel patches were being changed to use Netlink
attributes correctly some code was accidentially dropped which set all of the
undefined CIPSOv4 level and category mappings to a sentinel value.  The result
is the mappings data in the kernel contains bogus mappings which always map to
zero.  This patch restores the old/correct behavior by initializing the mapping
data to the correct sentinel value.

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