netlabel: correctly list all the static label mappings

When we have a large number of static label mappings that spill across
the netlink message boundary we fail to properly save our state in the
netlink_callback struct which causes us to repeat the same listings.
This patch fixes this problem by saving the state correctly between
calls to the NetLabel static label netlink "dumpit" routines.

Signed-off-by: Paul Moore <pmoore@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>