dccp: fix error in updating the GAR

This fixes a bug in updating the Greatest Acknowledgment number Received (GAR):
the current implementation does not track the greatest received value -
lower values in the range AWL..AWH (RFC 4340, 7.5.1) erase higher ones.
Signed-off-by: NGerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/dccp/input.c

@@ -239,7 +239,8 @@ static int dccp_check_seqno(struct sock *sk, struct sk_buff *skb)
 		dccp_update_gsr(sk, seqno);
 
 		if (dh->dccph_type != DCCP_PKT_SYNC &&
-		    (ackno != DCCP_PKT_WITHOUT_ACK_SEQ))
+		    ackno != DCCP_PKT_WITHOUT_ACK_SEQ &&
+		    after48(ackno, dp->dccps_gar))
 			dp->dccps_gar = ackno;
 	} else {
 		unsigned long now = jiffies;