selftests: mptcp: userspace_pm: use correct server port

"server4_port" variable is not set but "app4_port" is the server port in
v4 and the correct variable name to use.

The port is optional so there was no visible impact.

Link: multipath-tcp/mptcp_net-next#368
Fixes: ca188a2 ("selftests: mptcp: userspace PM support for MP_PRIO signals")
Cc: stable@vger.kernel.org
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

Author: matttbe

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

@@ -848,7 +848,7 @@ test_prio()
 	local count
 
 	# Send MP_PRIO signal from client to server machine
-	ip netns exec "$ns2" ./pm_nl_ctl set 10.0.1.2 port "$client4_port" flags backup token "$client4_token" rip 10.0.1.1 rport "$server4_port"
+	ip netns exec "$ns2" ./pm_nl_ctl set 10.0.1.2 port "$client4_port" flags backup token "$client4_token" rip 10.0.1.1 rport "$app4_port"
 	sleep 0.5
 
 	# Check TX