nfp: refuse offloading filters that redirects to upper devices

Previously we did not ensure that a netdev is a representative netdev
before dereferencing its private data. This can occur when an upper netdev
is created on a representative netdev. This patch corrects this by first
ensuring that the netdev is a representative netdev before using it.
Checking only switchdev_port_same_parent_id is not sufficient to ensure
that we can safely use the netdev. Failing to check that the netdev is also
a representative netdev would result in incorrect dereferencing.

Fixes: 1a1e586f ("nfp: add basic action capabilities to flower offloads")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Pieter Jansen van Vuuren <pieter.jansenvanvuuren@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>