forbid also fork

chapters/chapter28/seccomp/main.c

@@ -40,9 +40,10 @@ static int install_filter(int nr, int arch, int error) {
 int main(int argc, char *argv[]) {
 	pid_t id;
 
-	printf("Test sandbox!\n");
+	printf("Enable sandbox!\n");
 
 	install_filter(__NR_clone, AUDIT_ARCH_X86_64, EPERM);
+	install_filter(__NR_fork, AUDIT_ARCH_X86_64, EPERM);
 
 	id = fork();
 	if (id == -1) {