bootstrap: tar: use the nr_written value returned by xpc_buffer_write to increment the file cursor

sys/bootstrap/tar.c

@@ -191,7 +191,7 @@ static enum fs_status file_read(
 	size_t w;
 	xpc_buffer_write(buf, src, count, &w);
 
-	offset += count;
+	offset += w;
 	*seek = offset;
 
 	return FS_SUCCESS;