dccp: fix return value for sequence-invalid packets

Currently dccp_check_seqno returns 0 (indicating a valid packet) if the
acknowledgment number is out of bounds and the sync that RFC 4340 mandates at
this point is currently being rate-limited. This function should return -1,
indicating an invalid packet.
Signed-off-by: Samuel Jero <sj323707@ohio.edu>
Acked-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>

net/dccp/input.c

@@ -260,7 +260,7 @@ static int dccp_check_seqno(struct sock *sk, struct sk_buff *skb)
 		 */
 		if (time_before(now, (dp->dccps_rate_last +
 				      sysctl_dccp_sync_ratelimit)))
-			return 0;
+			return -1;
 
 		DCCP_WARN("Step 6 failed for %s packet, "
 			  "(LSWL(%llu) <= P.seqno(%llu) <= S.SWH(%llu)) and "