mptcp: avoid printing warning once on client side

After the 'Fixes' commit mentioned below, the client side might print
the following warning once when a subflow is fully established at the
reception of any valid additional ack:

  MPTCP: bogus mpc option on established client sk

That's a normal situation, and no warning should be printed for that. We
can then skip the check when the label is used.

Fixes: e4a0fa4 ("mptcp: corner case locking for rx path fields initialization")
Cc: stable@vger.kernel.org
Suggested-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240223-upstream-net-20240223-misc-fixes-v1-3-162e87e48497@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/options.c

@@ -981,10 +981,10 @@ static bool check_fully_established(struct mptcp_sock *msk, struct sock *ssk,
 	if (mp_opt->deny_join_id0)
 		WRITE_ONCE(msk->pm.remote_deny_join_id0, true);
 
-set_fully_established:
 	if (unlikely(!READ_ONCE(msk->pm.server_side)))
 		pr_warn_once("bogus mpc option on established client sk");
 
+set_fully_established:
 	mptcp_data_lock((struct sock *)msk);
 	__mptcp_subflow_fully_established(msk, subflow, mp_opt);
 	mptcp_data_unlock((struct sock *)msk);