timer: Remove redundant __setup_timer*() macros

With __init_timer*() now matching __setup_timer*(), remove the redundant
internal interface, clean up the resulting definitions and add more
documentation.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tejun Heo <tj@kernel.org>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Shaohua Li <shli@fb.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
diff --git a/include/linux/kthread.h b/include/linux/kthread.h
index b855c5b..dc850d2 100644
--- a/include/linux/kthread.h
+++ b/include/linux/kthread.h
@@ -164,9 +164,9 @@ extern void __kthread_init_worker(struct kthread_worker *worker,
 #define kthread_init_delayed_work(dwork, fn)				\
 	do {								\
 		kthread_init_work(&(dwork)->work, (fn));		\
-		__setup_timer(&(dwork)->timer,				\
-			      (TIMER_FUNC_TYPE)kthread_delayed_work_timer_fn,\
-			      TIMER_IRQSAFE);				\
+		__init_timer(&(dwork)->timer,				\
+			     kthread_delayed_work_timer_fn,		\
+			     TIMER_IRQSAFE);				\
 	} while (0)
 
 int kthread_worker_fn(void *worker_ptr);