Treat incomplete checksum.

Checksum of local traffic is often offloaded to the network device.
This causes some problems on parsing of this traffic. This patch
introduces a PKT_INCOMPLETE_CHECKSUM flag which can be used to
indicate that the checksum is not computed/correct for good reason.

src/decode.h

@@ -879,6 +879,8 @@ void AddressDebugPrint(Address *);
 #define PKT_TUNNEL                      0x1000
 #define PKT_TUNNEL_VERDICTED            0x2000
 
+#define PKT_IGNORE_CHECKSUM         0x4000    /**< Packet checksum is not computed (TX packet for example) */
+
 /** \brief return 1 if the packet is a pseudo packet */
 #define PKT_IS_PSEUDOPKT(p) ((p)->flags & PKT_PSEUDO_STREAM_END)
 

src/stream-tcp.c

@@ -3828,6 +3828,9 @@ static inline int StreamTcpValidateChecksum(Packet *p)
 {
     int ret = 1;
 
+    if (p->flags & PKT_IGNORE_CHECKSUM)
+        return ret;
+
     if (p->tcpvars.comp_csum == -1) {
         if (PKT_IS_IPV4(p)) {
             p->tcpvars.comp_csum = TCPCalculateChecksum((uint16_t *)&(p->ip4h->ip_src),