CVE-2016-2111: s3:rpc_server/netlogon: require DCERPC_AUTH_LEVEL_PRIVACY for validation level 6

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11749

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Günther Deschner <gd@samba.org>
