ethtool: wire up get policies to ops

Wire up policies for get commands in struct nla_policy of the ethtool
family. Make use of genetlink code attr validation and parsing, as well
as allow dumping policies to user space.

For every ETHTOOL_MSG_*_GET:
 - add 'ethnl_' prefix to policy name
 - add extern declaration in net/ethtool/netlink.h
 - wire up the policy & attr in ethtool_genl_ops[].
 - remove .request_policy and .max_attr from ethnl_request_ops.

Obviously core only records the first "layer" of parsed attrs
so we still need to parse the sub-attrs of the nested header
attribute.

v2:
 - merge of patches 1 and 2 from v1
 - remove stray empty lines in ops
 - also remove .max_attr

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>