Merge branch 'ethtool-extack'

Jakub Kicinski says:

====================
net: support extack in dump and simplify ethtool uAPI

Ethtool currently requires header nest to be always present even if
it doesn't have to carry any attr for a given request. This inflicts
unnecessary pain on the users.

What makes it worse is that extack was not working in dump's ->start()
callback. Address both of those issues.
====================

Signed-off-by: David S. Miller <davem@davemloft.net>