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/upd.c b/drivers/mtd/ubi/upd.c
index a95dcaa..e32b04d 100644
--- a/drivers/mtd/ubi/upd.c
+++ b/drivers/mtd/ubi/upd.c
@@ -67,7 +67,9 @@
 	memcpy(&vtbl_rec, &ubi->vtbl[vol_id], sizeof(struct ubi_vtbl_record));
 	vtbl_rec.upd_marker = 1;
 
+	mutex_lock(&ubi->volumes_mutex);
 	err = ubi_change_vtbl_record(ubi, vol_id, &vtbl_rec);
+	mutex_unlock(&ubi->volumes_mutex);
 	vol->upd_marker = 1;
 	return err;
 }
@@ -106,7 +108,9 @@
 			vol->last_eb_bytes = vol->usable_leb_size;
 	}
 
+	mutex_lock(&ubi->volumes_mutex);
 	err = ubi_change_vtbl_record(ubi, vol_id, &vtbl_rec);
+	mutex_unlock(&ubi->volumes_mutex);
 	vol->upd_marker = 0;
 	return err;
 }