[DCCP]: Handle SYNC packets in dccp_rcv_state_process

Eliciting a SYNCACK in response, we were handling SYNC packets
only in the DCCP_OPEN state, in dccp_rcv_established.

Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

net/dccp/input.c

···
 		return 0;
 	}
 
+	if (unlikely(dh->dccph_type == DCCP_PKT_SYNC)) {
+		dccp_send_sync(sk, DCCP_SKB_CB(skb)->dccpd_seq,
+			       DCCP_PKT_SYNCACK);
+		goto discard;
+	}
+
 	switch (sk->sk_state) {
 	case DCCP_CLOSED:
 		return 1;