dccp: fix bug in updating the GSR

Currently dccp_check_seqno allows any valid packet to update the Greatest
Sequence Number Received, even if that packet's sequence number is less than
the current GSR. This patch adds a check to make sure that the new packet's
sequence number is greater than GSR.
Signed-off-by: Samuel Jero <sj323707@ohio.edu>
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>

net/dccp/dccp.h

@@ -426,7 +426,8 @@ static inline void dccp_update_gsr(struct sock *sk, u64 seq)
 {
 	struct dccp_sock *dp = dccp_sk(sk);
 
-	dp->dccps_gsr = seq;
+	if (after48(seq, dp->dccps_gsr))
+		dp->dccps_gsr = seq;
 	/* Sequence validity window depends on remote Sequence Window (7.5.1) */
 	dp->dccps_swl = SUB48(ADD48(dp->dccps_gsr, 1), dp->dccps_r_seq_win / 4);
 	/*