kthread: Don't use to_live_kthread() in kthread_stop()

kthread_stop() had to use to_live_kthread() simply because it was not
possible to access kthread->exited after the exiting task clears
task_struct->vfork_done. Now that to_kthread() is always valid,
wake_up_process() + wait_for_completion() can be done
ununconditionally. It's not an issue anymore if the task has already issued
complete_vfork_done() or died.

The exiting task can get the spurious wakeup after mm_release() but this is
possible without this change too and is fine; do_task_dead() ensures that
this can't make any harm.

As a further enhancement this could be converted to task_work_add() later,
so ->vfork_done can be avoided completely.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Chunming Zhou <David1.Zhou@amd.com>
Cc: Roman Pen <roman.penyaev@profitbricks.com>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Tejun Heo <tj@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20161129175103.GA5336@redhat.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/kthread.c

@@ -532,13 +532,11 @@ int kthread_stop(struct task_struct *k)
 	trace_sched_kthread_stop(k);
 
 	get_task_struct(k);
-	kthread = to_live_kthread(k);
-	if (kthread) {
-		set_bit(KTHREAD_SHOULD_STOP, &kthread->flags);
-		__kthread_unpark(k, kthread);
-		wake_up_process(k);
-		wait_for_completion(&kthread->exited);
-	}
+	kthread = to_kthread(k);
+	set_bit(KTHREAD_SHOULD_STOP, &kthread->flags);
+	__kthread_unpark(k, kthread);
+	wake_up_process(k);
+	wait_for_completion(&kthread->exited);
 	ret = k->exit_code;
 	put_task_struct(k);