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/build.c b/drivers/mtd/ubi/build.c
index e0e8f47..5d8fda1 100644
--- a/drivers/mtd/ubi/build.c
+++ b/drivers/mtd/ubi/build.c
@@ -806,7 +806,7 @@
 
 	mutex_init(&ubi->buf_mutex);
 	mutex_init(&ubi->ckvol_mutex);
-	mutex_init(&ubi->volumes_mutex);
+	mutex_init(&ubi->device_mutex);
 	spin_lock_init(&ubi->volumes_lock);
 
 	ubi_msg("attaching mtd%d to ubi%d", mtd->index, ubi_num);