dcbnl: Add netlink attributes for setapp/getapp to dcbnl

Add defines for dcbnl netlink attributes to support netlink message passing of
setapp/getapp in dcbnl.

Signed-off-by: Yi Zou <yi.zou@intel.com>
Acked-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>