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: e06959e9 ("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>