filter-rewriter: skip net_checksum_calculate() while offset = 0

While the offset of packets's sequence for primary side and
secondary side is zero, it is unnecessary to call net_checksum_calculate()
to recalculate the checksume value of packets.
Signed-off-by: Nzhanghailiang <zhang.zhanghailiang@huawei.com>
Signed-off-by: NJason Wang <jasowang@redhat.com>

net/filter-rewriter.c

@@ -93,10 +93,12 @@ static int handle_primary_tcp_pkt(NetFilterState *nf,
             conn->offset -= (ntohl(tcp_pkt->th_ack) - 1);
             conn->syn_flag = 0;
         }
-        /* handle packets to the secondary from the primary */
-        tcp_pkt->th_ack = htonl(ntohl(tcp_pkt->th_ack) + conn->offset);
+        if (conn->offset) {
+            /* handle packets to the secondary from the primary */
+            tcp_pkt->th_ack = htonl(ntohl(tcp_pkt->th_ack) + conn->offset);
 
-        net_checksum_calculate((uint8_t *)pkt->data, pkt->size);
+            net_checksum_calculate((uint8_t *)pkt->data, pkt->size);
+        }
     }
 
     return 0;
@@ -129,10 +131,13 @@ static int handle_secondary_tcp_pkt(NetFilterState *nf,
     }
 
     if ((tcp_pkt->th_flags & (TH_ACK | TH_SYN)) == TH_ACK) {
-        /* handle packets to the primary from the secondary*/
-        tcp_pkt->th_seq = htonl(ntohl(tcp_pkt->th_seq) - conn->offset);
+        /* Only need to adjust seq while offset is Non-zero */
+        if (conn->offset) {
+            /* handle packets to the primary from the secondary*/
+            tcp_pkt->th_seq = htonl(ntohl(tcp_pkt->th_seq) - conn->offset);
 
-        net_checksum_calculate((uint8_t *)pkt->data, pkt->size);
+            net_checksum_calculate((uint8_t *)pkt->data, pkt->size);
+        }
     }
 
     return 0;