selftests: forwarding: Add missing 'rp_filter' configuration

When 'rp_filter' is configured in strict mode (1) the tests fail because
packets received from the macvlan netdevs would not be forwarded through
them on the reverse path.

Fix this by disabling the 'rp_filter', meaning no source validation is
performed.

Fixes: 1538812e ("selftests: forwarding: Add a test for VXLAN asymmetric routing")
Fixes: 438a4f56

 ("selftests: forwarding: Add a test for VXLAN symmetric routing")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reported-by: Hangbin Liu <liuhangbin@gmail.com>
Tested-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://lore.kernel.org/r/20201015084525.135121-1-idosch@idosch.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>