selftests: mptcp: join: mark implicit tests as skipped if not supported

The call to 'continue_if' was missing: it properly marks a subtest as
'skipped' if the attached condition is not valid.

Without that, the test is wrongly marked as passed on older kernels.

Fixes: 36c4127 ("selftests: mptcp: join: skip implicit tests if not supported")
Cc: stable@vger.kernel.org
Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20251020-net-mptcp-c-flag-late-add-addr-v1-3-8207030cb0e8@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: matttbe

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

@@ -3939,7 +3939,7 @@ endpoint_tests()
 	# subflow_rebuild_header is needed to support the implicit flag
 	# userspace pm type prevents add_addr
 	if reset "implicit EP" &&
-	   mptcp_lib_kallsyms_has "subflow_rebuild_header$"; then
+	   continue_if mptcp_lib_kallsyms_has "subflow_rebuild_header$"; then
 		pm_nl_set_limits $ns1 2 2
 		pm_nl_set_limits $ns2 2 2
 		pm_nl_add_endpoint $ns1 10.0.2.1 flags signal
@@ -3964,7 +3964,7 @@ endpoint_tests()
 	fi
 
 	if reset_with_tcp_filter "delete and re-add" ns2 10.0.3.2 REJECT OUTPUT &&
-	   mptcp_lib_kallsyms_has "subflow_rebuild_header$"; then
+	   continue_if mptcp_lib_kallsyms_has "subflow_rebuild_header$"; then
 		start_events
 		pm_nl_set_limits $ns1 0 3
 		pm_nl_set_limits $ns2 0 3