selftests: forwarding: mirror_gre_nh: Unset RP filter

The test fails to work if reverse-path filtering is in effect on the
mirrored-to host interface, or for all interfaces.

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