decode/ipv4: add missing ip-in-ip case handling

A flow with IPv4 IP in IP traffic won't handle this tunneling case
properly.
This leads to potential malicious traffic not triggering alerts, as well
as other inaccuracies in the logs.

Bug #7725

src/decode-ipv4.c

@@ -595,6 +595,16 @@ int DecodeIPV4(ThreadVars *tv, DecodeThreadVars *dtv, Packet *p,
             FlowSetupPacket(p);
             break;
         }
+        case IPPROTO_IPIP: {
+            /* spawn off tunnel packet */
+            Packet *tp = PacketTunnelPktSetup(tv, dtv, p, data, data_len, DECODE_TUNNEL_IPV4);
+            if (tp != NULL) {
+                PKT_SET_SRC(tp, PKT_SRC_DECODER_IPV4);
+                PacketEnqueueNoLock(&tv->decode_pq, tp);
+            }
+            FlowSetupPacket(p);
+            break;
+        }
         case IPPROTO_IP:
             /* check PPP VJ uncompressed packets and decode tcp dummy */
             if (p->flags & PKT_PPP_VJ_UCOMP) {