router: skip rt_tables write in alloc_table during --show dry-run

router/route-update.sh

@@ -159,7 +159,7 @@ alloc_table()
     local num=200
     while [ $num -le 250 ] ; do
         if ! echo "$used" | grep -q "^${num}$" ; then
-            echo "$num	$name" >> /etc/iproute2/rt_tables
+            [ -z "$SHOW" ] && echo "$num	$name" >> /etc/iproute2/rt_tables
             vlog "Allocated table $num for $name"
             echo "$num"
             return