devlink: have genetlink code to parse the attrs during dumpit

devlink: have genetlink code to parse the attrs during dumpit

Benefit from the fact that the generic netlink code can parse the attrs
for dumpit op and avoid need to parse it in the op callback.

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