selftests: mptcp: fix case multiple subflows limited by server

After patch "mptcp: fix syncookie process if mptcp can not_accept new
subflow", if subflow is limited, MP_JOIN SYN is dropped, and no SYN/ACK
will be replied.

So in case "multiple subflows limited by server", the expected SYN/ACK
number should be 1.

Fixes: 00587187 ("selftests: mptcp: add test cases for mptcp join tests with syn cookies")
Reported-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Jianguo Wu <wujianguo@chinatelecom.cn>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>