wireguard: selftests: make sure rp_filter is disabled on vethc

Some distros may enable strict rp_filter by default, which will prevent
vethc from receiving the packets with an unrouteable reverse path address.

Reported-by: Hangbin Liu <liuhangbin@gmail.com>
Fixes: e7096c13

 ("net: WireGuard secure network tunnel")
Cc: stable@vger.kernel.org
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: David S. Miller <davem@davemloft.net>