Fix error in cfs_alloc_page

Author: knizhnik

src/backend/storage/file/cfs.c

@@ -479,6 +479,8 @@ uint32 cfs_alloc_page(FileMap* map, uint32 oldSize, uint32 newSize)
 			elog(ERROR, "CFS: segment file exceed 4Gb limit");
 	} while (!pg_atomic_compare_exchange_u32(&map->hdr.physSize, &oldPhysSize, newPhysSize));
 
+	pg_atomic_fetch_sub_u32(&map->hdr.usedSize, newSize - oldSize);
+
 	return oldPhysSize;
 }