block: remove 'magic' from struct blk_plug

I don't think we've ever caught any bugs with this, and there's the
list poisoning for the plug lists to catch uninitialized cases.
So remove the magic member and save 8 bytes in the struct.
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-core.c

@@ -2957,8 +2957,6 @@ int kblockd_schedule_delayed_work_on(int cpu, struct delayed_work *dwork,
 }
 EXPORT_SYMBOL(kblockd_schedule_delayed_work_on);
 
-#define PLUG_MAGIC	0x91827364
-
 /**
  * blk_start_plug - initialize blk_plug and track it inside the task_struct
  * @plug:	The &struct blk_plug that needs to be initialized
@@ -2977,7 +2975,6 @@ void blk_start_plug(struct blk_plug *plug)
 {
 	struct task_struct *tsk = current;
 
-	plug->magic = PLUG_MAGIC;
 	INIT_LIST_HEAD(&plug->list);
 	INIT_LIST_HEAD(&plug->mq_list);
 	INIT_LIST_HEAD(&plug->cb_list);
@@ -3074,8 +3071,6 @@ void blk_flush_plug_list(struct blk_plug *plug, bool from_schedule)
 	LIST_HEAD(list);
 	unsigned int depth;
 
-	BUG_ON(plug->magic != PLUG_MAGIC);
-
 	flush_plug_callbacks(plug, from_schedule);
 
 	if (!list_empty(&plug->mq_list))

include/linux/blkdev.h

@@ -1060,7 +1060,6 @@ static inline void blk_post_runtime_resume(struct request_queue *q, int err) {}
  * schedule() where blk_schedule_flush_plug() is called.
  */
 struct blk_plug {
-	unsigned long magic; /* detect uninitialized use-cases */
 	struct list_head list; /* requests */
 	struct list_head mq_list; /* blk-mq requests */
 	struct list_head cb_list; /* md requires an unplug callback */