selftests: mptcp: join: mark 'flush re-add' 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: e06959e ("selftests: mptcp: join: test for flush/re-add endpoints")
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-v1-2-8207030cb0e8@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: matttbe

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

@@ -4115,7 +4115,7 @@ endpoint_tests()
 
 	# flush and re-add
 	if reset_with_tcp_filter "flush 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
 		pm_nl_set_limits $ns1 0 2
 		pm_nl_set_limits $ns2 1 2
 		# broadcast IP: no packet for this address will be received on ns1