blk-mq: move lockdep_assert_held() into elevator_exit

Commit c48dac13 ("block: don't hold q->sysfs_lock in elevator_init_mq")
removes q->sysfs_lock from elevator_init_mq(), but forgot to deal with
lockdep_assert_held() called in blk_mq_sched_free_requests() which is
run in failure path of elevator_init_mq().

blk_mq_sched_free_requests() is called in the following 3 functions:

	elevator_init_mq()
	elevator_exit()
	blk_cleanup_queue()

In blk_cleanup_queue(), blk_mq_sched_free_requests() is followed exactly
by 'mutex_lock(&q->sysfs_lock)'.

So moving the lockdep_assert_held() from blk_mq_sched_free_requests()
into elevator_exit() for fixing the report by syzbot.

Reported-by: syzbot+da3b7677bb913dc1b737@syzkaller.appspotmail.com
Fixed: c48dac13 ("block: don't hold q->sysfs_lock in elevator_init_mq")
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq-sched.c

@@ -555,8 +555,6 @@ void blk_mq_sched_free_requests(struct request_queue *q)
 	struct blk_mq_hw_ctx *hctx;
 	int i;
 
-	lockdep_assert_held(&q->sysfs_lock);
-
 	queue_for_each_hw_ctx(q, hctx, i) {
 		if (hctx->sched_tags)
 			blk_mq_free_rqs(q->tag_set, hctx->sched_tags, i);

block/blk.h

@@ -194,6 +194,8 @@ void elv_unregister_queue(struct request_queue *q);
 static inline void elevator_exit(struct request_queue *q,
 		struct elevator_queue *e)
 {
+	lockdep_assert_held(&q->sysfs_lock);
+
 	blk_mq_sched_free_requests(q);
 	__elevator_exit(q, e);
 }