UBI: re-name volumes_mutex to device_mutex

The mutex essencially protects the entire UBI device, so the
old @volumes_mutex name is a little misleading.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
diff --git a/drivers/mtd/ubi/ubi.h b/drivers/mtd/ubi/ubi.h
index 485c73f..76ec79b 100644
--- a/drivers/mtd/ubi/ubi.h
+++ b/drivers/mtd/ubi/ubi.h
@@ -305,9 +305,9 @@
  * @vtbl_slots: how many slots are available in the volume table
  * @vtbl_size: size of the volume table in bytes
  * @vtbl: in-RAM volume table copy
- * @volumes_mutex: protects on-flash volume table and serializes volume
- *                 changes, like creation, deletion, update, re-size,
- *                 re-name and set property
+ * @device_mutex: protects on-flash volume table and serializes volume
+ *                creation, deletion, update, re-size, re-name and set
+ *                property
  *
  * @max_ec: current highest erase counter value
  * @mean_ec: current mean erase counter value
@@ -388,7 +388,7 @@
 	int vtbl_slots;
 	int vtbl_size;
 	struct ubi_vtbl_record *vtbl;
-	struct mutex volumes_mutex;
+	struct mutex device_mutex;
 
 	int max_ec;
 	/* Note, mean_ec is not updated run-time - should be fixed */