UBI: Fastmap: Ensure that all fastmap work is done upon WL shutdown

...otherwise the deferred work might run after datastructures
got freed and corrupt memory.
Signed-off-by: Richard Weinberger <richard@nod.at>
Reviewed-by: Guido Martínez <guido@vanguardiasur.com.ar>

drivers/mtd/ubi/wl.c

@@ -1856,6 +1856,9 @@ int ubi_thread(void *u)
  */
 static void shutdown_work(struct ubi_device *ubi)
 {
+#ifdef CONFIG_MTD_UBI_FASTMAP
+	flush_work(&ubi->fm_work);
+#endif
 	while (!list_empty(&ubi->works)) {
 		struct ubi_work *wrk;