test: write_read_10 -- Fix typo in testing opened file descriptor

Should be @child_fd instead of @fd
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

test/zdtm/live/static/write_read10.c

@@ -32,7 +32,7 @@ int main(int argc, char ** argv)
 	}
 
 	child_fd = open(filename, O_RDWR | O_CREAT | O_TRUNC, 0644);
-	if (fd < 0) {
+	if (child_fd < 0) {
 		pr_perror("can't open %s", filename);
 		exit(1);
 	}