UBI: add dump_stack in checking code

I am experiencing an error in 'paranoid_check_volume()'. Add
dump_stack() there to make it easier to identify the reasons
of the error.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

drivers/mtd/ubi/vmt.c

@@ -868,6 +868,7 @@ fail:
 	if (vol)
 		ubi_dbg_dump_vol_info(vol);
 	ubi_dbg_dump_vtbl_record(&ubi->vtbl[vol_id], vol_id);
+	dump_stack();
 	spin_unlock(&ubi->volumes_lock);
 	return -EINVAL;
 }