UBI: tweak volumes locking

Transform vtbl_mutex to volumes_mutex - this just makes code
easier to understand.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
diff --git a/drivers/mtd/ubi/build.c b/drivers/mtd/ubi/build.c
index 5d00364..61225f4 100644
--- a/drivers/mtd/ubi/build.c
+++ b/drivers/mtd/ubi/build.c
@@ -264,7 +264,7 @@
 	int i, err;
 	dev_t dev;
 
-	mutex_init(&ubi->vtbl_mutex);
+	mutex_init(&ubi->volumes_mutex);
 	spin_lock_init(&ubi->volumes_lock);
 
 	sprintf(ubi->ubi_name, UBI_NAME_STR "%d", ubi->ubi_num);