[v11][PATCH 8/9] Define clone_with_pids() syscall

[Serge E. Hallyn]

Quoting Sukadev Bhattiprolu (sukadev at us.ibm.com):
> +	stack_size = (unsigned long)kca.child_stack_size;
> +	child_stack = (unsigned long)kca.child_stack_base + stack_size;
> +
> +	if (!child_stack)
> +		child_stack = regs->sp;

I'm hooking up the s390 version right now.  Do you think you should
make this

	if (!kca.child_stack_base)
		child_stack = regs->sp;

?

I suppose that in general if I pass in a NULL kca.child_stack_base
I'll also pass in a 0 stacksize, but as a user I'd expect that if
I pass in NULL, the size gets ignored.  Instead, if I pass in NULL
plus a size, then the kernel will take (void *)size as the stacktop.

-serge