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>

tools/testing/selftests/wireguard/netns.sh

@@ -363,6 +363,7 @@ ip1 -6 rule add table main suppress_prefixlength 0
 ip1 -4 route add default dev wg0 table 51820
 ip1 -4 rule add not fwmark 51820 table 51820
 ip1 -4 rule add table main suppress_prefixlength 0
+n1 bash -c 'printf 0 > /proc/sys/net/ipv4/conf/vethc/rp_filter'
 # Flood the pings instead of sending just one, to trigger routing table reference counting bugs.
 n1 ping -W 1 -c 100 -f 192.168.99.7
 n1 ping -W 1 -c 100 -f abab::1111