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>

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

@@ -1409,7 +1409,7 @@ syncookies_tests()
 	ip netns exec $ns2 ./pm_nl_ctl add 10.0.3.2 flags subflow
 	ip netns exec $ns2 ./pm_nl_ctl add 10.0.2.2 flags subflow
 	run_tests $ns1 $ns2 10.0.1.1
-	chk_join_nr "subflows limited by server w cookies" 2 2 1
+	chk_join_nr "subflows limited by server w cookies" 2 1 1
 
 	# test signal address with cookies
 	reset_with_cookies