selftests: mptcp: userspace pm create id 0 subflow

This patch adds a selftest to create id 0 subflow. Pass id 0 to the
helper userspace_pm_add_sf() to create id 0 subflow. chk_mptcp_info
shows one subflow but chk_all_subflows shows two subflows in each
namespace.

Signed-off-by: Geliang Tang <geliang.tang@suse.com>

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

@@ -3477,6 +3477,25 @@ userspace_tests()
 		wait $tests_pid
 	fi
 
+	# userspace pm create id 0 subflow
+	if reset_with_events "userspace pm create id 0 subflow" &&
+	   continue_if mptcp_lib_has_file '/proc/sys/net/mptcp/pm_type'; then
+		set_userspace_pm $ns2
+		pm_nl_set_limits $ns1 0 1
+		speed=10 \
+			run_tests $ns1 $ns2 10.0.1.1 &
+		local tests_pid=$!
+		wait_mpj $ns2
+		chk_mptcp_info subflows 0 subflows 0
+		chk_all_subflows 1 1
+		userspace_pm_add_sf $ns2 10.0.3.2 0
+		chk_join_nr 1 1 1
+		chk_mptcp_info subflows 1 subflows 1
+		chk_all_subflows 2 2
+		kill_events_pids
+		wait $tests_pid
+	fi
+
 	# userspace pm remove id 0 subflow
 	if reset_with_events "userspace pm remove id 0 subflow" &&
 	   continue_if mptcp_lib_has_file '/proc/sys/net/mptcp/pm_type'; then