We are leaving local interrupts disabled coming out of exit_notify().

But we are about to call wait_task_inactive() which spins, waiting for
another CPU to end a task.  If that CPU has issued smp_call_function() to
this CPU, deadlock.

So the patch enables interrupts again before returning from exit_notify().

Also, exit_notify() returns with preemption disabled, so there is no
need to perform another preempt_disable() in do_exit().