selftests: forwarding: Tweak tc filters for mirror-to-gretap tests

When running mirror_gre_bridge_1d_vlan tests on veth, several issues
cause spurious failures:

- vlan_ethtype should be ip, not ipv6 even in mirror-to-ip6gretap case,
  because the overlay packet is still IPv4.
- Similarly ip_proto matches the innermost IP protocol, so can't be used
  to filter out GRE packet. Drop the corresponding condition.
- Because the above fixes the filters to match in slow path as well,
  they need to be made skip_hw so as not to double-count packets.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>