stop_machine: Fix possible cpu_stopper_thread() crash

stop_one_cpu_nowait(fn) will crash the kernel if the callback returns
nonzero, work->done == NULL in this case.

This needs more cleanups, cpu_stop_signal_done() is called right after
we check done != NULL and it does the same check.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Tejun Heo <tj@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Milos Vyletel <milos@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20151115193311.GA8242@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/stop_machine.c

@@ -454,7 +454,7 @@ static void cpu_stopper_thread(unsigned int cpu)
 		preempt_disable();
 
 		ret = fn(arg);
-		if (ret)
+		if (ret && done)
 			done->ret = ret;
 
 		/* restore preemption and check it's still balanced */