selftests: mptcp: join: skip MPC backups tests if not supported

Selftests are supposed to run on any kernels, including the old ones not
supporting all MPTCP features.

One of them is the support of sending an MP_PRIO signal for the initial
subflow, introduced by commit c157bbe7 ("mptcp: allow the in kernel
PM to set MPC subflow priority").

It is possible to look for "mptcp_subflow_send_ack" in kallsyms because
it was needed to introduce the mentioned feature. So we can know in
advance if the feature is supported instead of trying and accepting any
results.

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/368
Fixes: 914f6a59 ("selftests: mptcp: add MPC backup tests")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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

@@ -2713,14 +2713,16 @@ backup_tests()
 		chk_prio_nr 1 1
 	fi
 
-	if reset "mpc backup"; then
+	if reset "mpc backup" &&
+	   continue_if mptcp_lib_kallsyms_doesnt_have "mptcp_subflow_send_ack$"; then
 		pm_nl_add_endpoint $ns2 10.0.1.2 flags subflow,backup
 		run_tests $ns1 $ns2 10.0.1.1 0 0 0 slow
 		chk_join_nr 0 0 0
 		chk_prio_nr 0 1
 	fi
 
-	if reset "mpc backup both sides"; then
+	if reset "mpc backup both sides" &&
+	   continue_if mptcp_lib_kallsyms_doesnt_have "mptcp_subflow_send_ack$"; then
 		pm_nl_add_endpoint $ns1 10.0.1.1 flags subflow,backup
 		pm_nl_add_endpoint $ns2 10.0.1.2 flags subflow,backup
 		run_tests $ns1 $ns2 10.0.1.1 0 0 0 slow
@@ -2728,14 +2730,16 @@ backup_tests()
 		chk_prio_nr 1 1
 	fi
 
-	if reset "mpc switch to backup"; then
+	if reset "mpc switch to backup" &&
+	   continue_if mptcp_lib_kallsyms_doesnt_have "mptcp_subflow_send_ack$"; then
 		pm_nl_add_endpoint $ns2 10.0.1.2 flags subflow
 		run_tests $ns1 $ns2 10.0.1.1 0 0 0 slow backup
 		chk_join_nr 0 0 0
 		chk_prio_nr 0 1
 	fi
 
-	if reset "mpc switch to backup both sides"; then
+	if reset "mpc switch to backup both sides" &&
+	   continue_if mptcp_lib_kallsyms_doesnt_have "mptcp_subflow_send_ack$"; then
 		pm_nl_add_endpoint $ns1 10.0.1.1 flags subflow
 		pm_nl_add_endpoint $ns2 10.0.1.2 flags subflow
 		run_tests $ns1 $ns2 10.0.1.1 0 0 0 slow backup