UBI: Fastmap: Switch to ro mode if invalidate_fastmap() fails

We have to switch to ro mode to guarantee that upon next UBI attach
all data is consistent.
Signed-off-by: NRichard Weinberger <richard@nod.at>

drivers/mtd/ubi/fastmap.c

@@ -1466,8 +1466,10 @@ int ubi_update_fastmap(struct ubi_device *ubi)
 	ret = 0;
 	if (old_fm) {
 		ret = invalidate_fastmap(ubi, old_fm);
-		if (ret < 0)
+		if (ret < 0) {
 			ubi_err(ubi, "Unable to invalidiate current fastmap!");
+			ubi_ro_mode(ubi);
+		}
 		else if (ret)
 			ret = 0;
 	}