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>