UBI: add UBI devices reference counting

This is one more step on the way to "removable" UBI devices. It
adds reference counting for UBI devices. Every time a volume on
this device is opened - the device's refcount is increased. It
is also increased if someone is reading any sysfs file of this
UBI device or of one of its volumes.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

drivers/mtd/ubi/wl.c

@@ -1303,6 +1303,7 @@ int ubi_wl_flush(struct ubi_device *ubi)
 	 * Make sure all the works which have been done in parallel are
 	 * finished.
 	 */
+	ubi_assert(ubi->ref_count > 0);
 	down_write(&ubi->work_sem);
 	up_write(&ubi->work_sem);