wait_task_inactive is now only used in two non-time-critical places: the
ptrace code to guarantee a schedule and kthread_bind so we can change
the thread CPU.  Unfortunately with preempt, the code as stands has a
race: we might return because the thread is preempted, not because it
actually reached schedule().  The ptrace code (probably) doesn't care,
but the kthread code does.

This patch simplifies the (now over-optimized) code, and does a yield()
for the preemption case.