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: https://github.com/multipath-tcp/mptcp_net-next/issues/368
Fixes: ca188a25

 ("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>