net/filter-rewriter.c: Fix rewirter checksum bug when use virtio-net

Because vnet_hdr have a offset to net packet, we must add it when use
virtio-net.

Signed-off-by: Zhang Chen <zhangchen.fnst@cn.fujitsu.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

net/filter-rewriter.c

@@ -99,7 +99,8 @@ static int handle_primary_tcp_pkt(NetFilterState *nf,
             /* 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->vnet_hdr_len,
+                                   pkt->size - pkt->vnet_hdr_len);
         }
     }
 
@@ -138,7 +139,8 @@ static int handle_secondary_tcp_pkt(NetFilterState *nf,
             /* 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->vnet_hdr_len,
+                                   pkt->size - pkt->vnet_hdr_len);
         }
     }