selftests: forwarding: mirror_gre_vlan_bridge_1q: Unset rp_filter

The IP addresses of tunnel endpoint at H3 are set at the VLAN device
$h3.555. Therefore when test_gretap_untagged_egress() sets vlan 555 to
egress untagged at $swp3, $h3's rp_filter rejects these packets. The
test then spuriously fails.

Therefore turn off net.ipv4.conf.{all, $h3}.rp_filter.

Fixes: 9c7c8a82 ("selftests: forwarding: mirror_gre_vlan_bridge_1q: Add more tests")
Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>