net: ethtool: Add infrastructure for reporting cable test results

Provide infrastructure for PHY drivers to report the cable test
results.  A netlink skb is associated to the phydev. Helpers will be
added which can add results to this skb. Once the test has finished
the results are sent to user space.

When netlink ethtool is not part of the kernel configuration stubs are
provided. It is also impossible to trigger a cable test, so the error
code returned by the alloc function is of no consequence.

v2:
Include the status complete in the netlink notification message

v4:
Replace -EINVAL with -EMSGSIZE

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>