selftests: forwarding: mirror_gre_nh: Unset rp_filter on host VRF

The mirrored packets arrive at $h3 encapsulated in GRE/IPv4, with IP
address from 192.0.2.128/28 network. However the interface is configured
as a member of 192.0.2.160/28 and there's no route directing traffic
from the former network through that interface. Correspondingly, the RP
filter on the VRF rejects it.

Therefore turn off the VRF's RP filter.

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