Make src-prefix action work in install filter

The purpose is to override *locally* RTA_SRC ('from' option in ip-route).

babeld.man

@@ -577,7 +577,7 @@ For a redistribute filter, redistribute this route with metric
 .IR value .
 .TP
 .BI src-prefix " prefix"
-For a redistribute filter, set the source prefix of this route to
+For a redistribute or install filter, set the source prefix of this route to
 .IR prefix .
 .TP
 .BI table " table"

route.c

@@ -444,23 +444,26 @@ change_route(int operation, const struct babel_route *route, int metric,
              int new_ifindex, int new_metric)
 {
     struct filter_result filter_result;
-    unsigned char *pref_src = NULL;
+    struct source *src = route->src;
+    unsigned char *pref_src = NULL, *src_prefix;
     unsigned int ifindex = route->neigh->ifp->ifindex;
     int m, table;
 
-    m = install_filter(route->src->prefix, route->src->plen,
-                       route->src->src_prefix, route->src->src_plen,
+    m = install_filter(src->prefix, src->plen,
+                       src->src_prefix, src->src_plen,
                        ifindex, &filter_result);
     if(m >= INFINITY && operation == ROUTE_ADD) {
         errno = EPERM;
         return -1;
     }
 
-    pref_src = filter_result.pref_src;
+    src_prefix = filter_result.src_prefix;
     table = filter_result.table ? filter_result.table : export_table;
 
-    return kernel_route(operation, table, route->src->prefix, route->src->plen,
-                        route->src->src_prefix, route->src->src_plen, pref_src,
+    return kernel_route(operation, table, src->prefix, src->plen,
+                        src_prefix ? src_prefix : src->src_prefix,
+                        src_prefix ? filter_result.src_plen : src->src_plen,
+                        filter_result.pref_src,
                         route->nexthop, ifindex,
                         metric, new_next_hop, new_ifindex, new_metric,
                         operation == ROUTE_MODIFY ? table : 0);