ubi: fastmap: Check each mapping only once

Maintain a bitmap to keep track of which LEB->PEB mapping
was checked already.
That way we have to read back VID headers only once.

Signed-off-by: Richard Weinberger <richard@nod.at>
diff --git a/drivers/mtd/ubi/vmt.c b/drivers/mtd/ubi/vmt.c
index 3fd8d7f..0be5167 100644
--- a/drivers/mtd/ubi/vmt.c
+++ b/drivers/mtd/ubi/vmt.c
@@ -139,6 +139,7 @@ static void vol_release(struct device *dev)
 	struct ubi_volume *vol = container_of(dev, struct ubi_volume, dev);
 
 	ubi_eba_replace_table(vol, NULL);
+	ubi_fastmap_destroy_checkmap(vol);
 	kfree(vol);
 }