ubi: fastmap: Only produce the initial anchor PEB when fastmap is used

Don't produce the initial anchor PEB when ubi device is read-only
or fastmap is disabled, else the resulting PEB will be unusable
to any volume.
Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

drivers/mtd/ubi/wl.c

@@ -1875,7 +1875,8 @@ int ubi_wl_init(struct ubi_device *ubi, struct ubi_attach_info *ai)
 		goto out_free;
 
 #ifdef CONFIG_MTD_UBI_FASTMAP
-	ubi_ensure_anchor_pebs(ubi);
+	if (!ubi->ro_mode && !ubi->fm_disabled)
+		ubi_ensure_anchor_pebs(ubi);
 #endif
 	return 0;