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/upd.c b/drivers/mtd/ubi/upd.c
index 6b4d1ae..dce1d92 100644
--- a/drivers/mtd/ubi/upd.c
+++ b/drivers/mtd/ubi/upd.c
@@ -68,9 +68,9 @@
 	       sizeof(struct ubi_vtbl_record));
 	vtbl_rec.upd_marker = 1;
 
-	mutex_lock(&ubi->volumes_mutex);
+	mutex_lock(&ubi->device_mutex);
 	err = ubi_change_vtbl_record(ubi, vol->vol_id, &vtbl_rec);
-	mutex_unlock(&ubi->volumes_mutex);
+	mutex_unlock(&ubi->device_mutex);
 	vol->upd_marker = 1;
 	return err;
 }
@@ -109,9 +109,9 @@
 			vol->last_eb_bytes = vol->usable_leb_size;
 	}
 
-	mutex_lock(&ubi->volumes_mutex);
+	mutex_lock(&ubi->device_mutex);
 	err = ubi_change_vtbl_record(ubi, vol->vol_id, &vtbl_rec);
-	mutex_unlock(&ubi->volumes_mutex);
+	mutex_unlock(&ubi->device_mutex);
 	vol->upd_marker = 0;
 	return err;
 }