flow: move livedev into Flow 'header'

It won't get updated after initial set.

src/flow.h

@@ -388,10 +388,11 @@ typedef struct Flow_
      *  the flow_id. */
     uint32_t flow_hash;
 
-    struct Flow_ *next; /* (hash) list next */
     /** Incoming interface */
     struct LiveDevice_ *livedev;
 
+    struct Flow_ *next; /* (hash) list next */
+
     /** timeout in seconds by policy, add to Flow::lastts to get actual time this times out.
      *  Ignored in emergency mode. */
     uint32_t timeout_policy;