The new definition of the clone flags is binary incompatible with older
2.5 kernels.
How about this instead:
#define CLONE_PARENT_SETTID  0x00100000
#define CLONE_CHILD_CLEARTID 0x00200000
#define CLONE_DETACHED	0x00400000
#define CLONE_UNTRACED       0x00800000
#define CLONE_CHILD_SETTID   0x01000000

> -#if CONFIG_SMP || CONFIG_PREEMPT
> +asmlinkage void FASTCALL(schedule_tail(task_t *prev));
>  asmlinkage void schedule_tail(task_t *prev)
>  {
>  	finish_arch_switch(this_rq(), prev);
Maybe finish_arch_switch should only be called if CONFIG_SMP ||
CONFIG_PREEMPT, like what happened without this patch?

> +	if (clone_flags & CLONE_PARENT_SETTID)
> +		put_user(p->pid, parent_tidptr);
How about failing if put_user fails?