[DCCP] options: Fix some aspects of mandatory option processing

According to dccp draft (draft-ietf-dccp-spec-13.txt) section 5.8.2
(Mandatory Option) the following patch correct the handling of the
following cases:

1) "... and any Mandatory options received on DCCP-Data packets MUST be
  ignored."

2) "The connection is in error and should be reset with Reset Code 5, ...
  if option O is absent (Mandatory was the last byte of the option list), or
  if option O equals Mandatory."
Signed-off-by: NArnaldo Carvalho de Melo <acme@mandriva.com>
Signed-off-by: NHagen Paul Pfeifer <hagen@jauu.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/dccp/options.c

@@ -109,7 +109,8 @@ int dccp_parse_options(struct sock *sk, struct sk_buff *skb)
 		case DCCPO_MANDATORY:
 			if (mandatory)
 				goto out_invalid_option;
-			mandatory = 1;
+			if (pkt_type != DCCP_PKT_DATA)
+				mandatory = 1;
 			break;
 		case DCCPO_NDP_COUNT:
 			if (len > 3)
@@ -249,6 +250,10 @@ int dccp_parse_options(struct sock *sk, struct sk_buff *skb)
 			mandatory = 0;
 	}
 
+	/* mandatory was the last byte in option list -> reset connection */
+	if (mandatory)
+		goto out_invalid_option;
+
 	return 0;
 
 out_invalid_option: