ethtool: helper functions for netlink interface

Add common request/reply header definition and helpers to parse request
header and fill reply header. Provide ethnl_update_* helpers to update
structure members from request attributes (to be used for *_SET requests).

Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>