selftests/bpf: Set gopt opt_class to 0 if get tunnel opt failed

When fixing the bpf test_tunnel.sh geneve failure. I only fixed the IPv4
part but forgot the IPv6 issue. Similar with the IPv4 fixes 557c223b
("selftests/bpf: No need to drop the packet when there is no geneve opt"),
when there is no tunnel option and bpf_skb_get_tunnel_opt() returns error,
there is no need to drop the packets and break all geneve rx traffic.
Just set opt_class to 0 and keep returning TC_ACT_OK at the end.

Fixes: 557c223b ("selftests/bpf: No need to drop the packet when there is no geneve opt")
Fixes: 933a741e ("selftests/bpf: bpf tunnel test.")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: William Tu <u9012063@gmail.com>
Link: https://lore.kernel.org/bpf/20210309032214.2112438-1-liuhangbin@gmail.com