mptcp: relax check on MPC passive fallback

While testing the blamed commit below, I was able to miss (!)
packetdrill failures in the fastopen test-cases.

On passive fastopen the child socket is created by incoming TCP MPC syn,
allow for both MPC_SYN and MPC_ACK header.

Fixes: 724b00c1 ("mptcp: refine opt_mp_capable determination")
Reviewed-by: Matthieu Baerts <matttbe@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/subflow.c

@@ -783,7 +783,8 @@ static struct sock *subflow_syn_recv_sock(const struct sock *sk,
 		 * options.
 		 */
 		mptcp_get_options(skb, &mp_opt);
-		if (!(mp_opt.suboptions & OPTION_MPTCP_MPC_ACK))
+		if (!(mp_opt.suboptions &
+		      (OPTION_MPTCP_MPC_SYN | OPTION_MPTCP_MPC_ACK)))
 			fallback = true;
 
 	} else if (subflow_req->mp_join) {