selftests: mptcp: userspace_pm: fix undefined variable port

selftests: mptcp: userspace_pm: fix undefined variable port

In make_connection(), the variable "port" is used but never defined.
This leads to an empty argument being passed to wait_local_port_listen(),
causing "printf: : invalid number" errors:

 # INFO: Init
 # 01 Created network namespaces ns1, ns2                          [ OK ]
 # INFO: Make connections
 # ./../lib.sh: line 651: printf: : invalid number
 # 02 Established IPv4 MPTCP Connection ns2 => ns1                 [ OK ]
 # INFO: Connection info: 10.0.1.2:59516 -> 10.0.1.1:50002
 # ./../lib.sh: line 651: printf: : invalid number
 # 03 Established IPv6 MPTCP Connection ns2 => ns1                 [ OK ]

Fix it by using the correctly defined variable "app_port", which holds the
appropriate port number for the connection.

Fixes: 39348f5f2f13 ("selftests: mptcp: wait for port instead of sleep")
Cc: stable@vger.kernel.org
Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260722-net-mptcp-misc-fixes-7-2-rc5-v1-4-6fb595bc86ef@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>