selftests: mptcp: enable syncookie only in absence of reorders

Syncookie validation may fail for OoO packets, causing spurious
resets and self-tests failures, so let's force syncookie only
for tests iteration with no OoO.

Fixes: fed61c4 ("selftests: mptcp: make 2nd net namespace use tcp syn cookies unconditionally")
Closes: #198
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

tools/testing/selftests/net/mptcp/mptcp_connect.sh

@@ -197,9 +197,6 @@ ip -net "$ns4" link set ns4eth3 up
 ip -net "$ns4" route add default via 10.0.3.2
 ip -net "$ns4" route add default via dead:beef:3::2
 
-# use TCP syn cookies, even if no flooding was detected.
-ip netns exec "$ns2" sysctl -q net.ipv4.tcp_syncookies=2
-
 set_ethtool_flags() {
 	local ns="$1"
 	local dev="$2"
@@ -737,6 +734,14 @@ for sender in $ns1 $ns2 $ns3 $ns4;do
 		exit $ret
 	fi
 
+	# ns1<->ns2 is not subject to reordering/tc delays. Use it to test
+	# mptcp syncookie support.
+	if [ $sender = $ns1 ]; then
+		ip netns exec "$ns2" sysctl -q net.ipv4.tcp_syncookies=2
+	else
+		ip netns exec "$ns2" sysctl -q net.ipv4.tcp_syncookies=1
+	fi
+
 	run_tests "$ns2" $sender 10.0.1.2
 	run_tests "$ns2" $sender dead:beef:1::2
 	run_tests "$ns2" $sender 10.0.2.1