mptcp: avoid a WARN on bad input.

Syzcaller has found a way to trigger the WARN_ON_ONCE condition
in check_fully_established().

The root cause is a legit fallback to TCP scenario, so replace
the WARN with a plain message on a more strict condition.

Fixes: f296234c ("mptcp: Add handling of incoming MP_JOIN requests")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/options.c

@@ -703,8 +703,6 @@ static bool check_fully_established(struct mptcp_sock *msk, struct sock *sk,
 		goto fully_established;
 	}
 
-	WARN_ON_ONCE(subflow->can_ack);
-
 	/* If the first established packet does not contain MP_CAPABLE + data
 	 * then fallback to TCP
 	 */
@@ -714,6 +712,8 @@ static bool check_fully_established(struct mptcp_sock *msk, struct sock *sk,
 		return false;
 	}
 
+	if (unlikely(!READ_ONCE(msk->pm.server_side)))
+		pr_warn_once("bogus mpc option on established client sk");
 	subflow->fully_established = 1;
 	subflow->remote_key = mp_opt->sndr_key;
 	subflow->can_ack = 1;