mptcp: fix 'use_ack' option access.

The mentioned RX option field is initialized only for DSS
packet, we must access it only if 'dss' is set too, or
the subflow will end-up in a bad status, leading to
RFC violations.

Fixes: d22f4988 ("mptcp: process MP_CAPABLE data option")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/options.c

@@ -695,7 +695,7 @@ static bool check_fully_established(struct mptcp_sock *msk, struct sock *sk,
 	if (TCP_SKB_CB(skb)->seq != subflow->ssn_offset + 1)
 		return subflow->mp_capable;
 
-	if (mp_opt->use_ack) {
+	if (mp_opt->dss && mp_opt->use_ack) {
 		/* subflows are fully established as soon as we get any
 		 * additional ack.
 		 */