tcp: account for ts offset only if tsecr not zero

We can get SYN with zero tsecr, don't apply offset in this case.

Fixes: ee684b6f ("tcp: send packets with a socket timestamp")
Signed-off-by: Alexey Kodanev <alexey.kodanev@oracle.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_minisocks.c

@@ -101,7 +101,8 @@ tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,
 		tcp_parse_options(skb, &tmp_opt, 0, NULL);
 
 		if (tmp_opt.saw_tstamp) {
-			tmp_opt.rcv_tsecr	-= tcptw->tw_ts_offset;
+			if (tmp_opt.rcv_tsecr)
+				tmp_opt.rcv_tsecr -= tcptw->tw_ts_offset;
 			tmp_opt.ts_recent	= tcptw->tw_ts_recent;
 			tmp_opt.ts_recent_stamp	= tcptw->tw_ts_recent_stamp;
 			paws_reject = tcp_paws_reject(&tmp_opt, th->rst);