net: genetlink: parse attrs and store in contect info struct during dumpit

Extend the dumpit info struct for attrs. Instead of existing attribute
validation do parse them and save in the info struct. Caller can benefit
from this and does not have to do parse itself. In order to properly
free attrs, genl_family pointer needs to be added to dumpit info struct
as well.

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