devlink: Add IPv6 header for dpipe

This will be used by the IPv6 host table which will be introduced in the
following patches. The fields in the header are added per-use. This header
is global and can be reused by many drivers.

Signed-off-by: Arkadi Sharshevsky <arkadis@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>