ethtool: add a stricter length check

There has been a few errors in the ethtool reply size calculations,
most of those are hard to trigger during basic testing because of
skb size rounding up and netdev names being shorter than max.
Add a more precise check.

This change will affect the value of payload length displayed in
case of -EMSGSIZE but that should be okay, "payload length" isn't
a well defined term here.

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