selftests/bpf: check for chain-non-0 extack message

Make sure netdevsim doesn't allow offload of chains other than 0,
and that it reports the expected extack message.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>