block: Fix a lockdep complaint triggered by request queue flushing

Avoid that running test nvme/012 from the blktests suite triggers the
following false positive lockdep complaint:

============================================
WARNING: possible recursive locking detected
5.0.0-rc3-xfstests-00015-g1236f7d60242 #841 Not tainted
--------------------------------------------
ksoftirqd/1/16 is trying to acquire lock:
000000000282032e (&(&fq->mq_flush_lock)->rlock){..-.}, at: flush_end_io+0x4e/0x1d0

but task is already holding lock:
00000000cbadcbc2 (&(&fq->mq_flush_lock)->rlock){..-.}, at: flush_end_io+0x4e/0x1d0

other info that might help us debug this:
 Possible unsafe locking scenario:

       CPU0
       ----
  lock(&(&fq->mq_flush_lock)->rlock);
  lock(&(&fq->mq_flush_lock)->rlock);

 *** DEADLOCK ***

 May be due to missing lock nesting notation

1 lock held by ksoftirqd/1/16:
 #0: 00000000cbadcbc2 (&(&fq->mq_flush_lock)->rlock){..-.}, at: flush_end_io+0x4e/0x1d0

stack backtrace:
CPU: 1 PID: 16 Comm: ksoftirqd/1 Not tainted 5.0.0-rc3-xfstests-00015-g1236f7d60242 #841
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 dump_stack+0x67/0x90
 __lock_acquire.cold.45+0x2b4/0x313
 lock_acquire+0x98/0x160
 _raw_spin_lock_irqsave+0x3b/0x80
 flush_end_io+0x4e/0x1d0
 blk_mq_complete_request+0x76/0x110
 nvmet_req_complete+0x15/0x110 [nvmet]
 nvmet_bio_done+0x27/0x50 [nvmet]
 blk_update_request+0xd7/0x2d0
 blk_mq_end_request+0x1a/0x100
 blk_flush_complete_seq+0xe5/0x350
 flush_end_io+0x12f/0x1d0
 blk_done_softirq+0x9f/0xd0
 __do_softirq+0xca/0x440
 run_ksoftirqd+0x24/0x50
 smpboot_thread_fn+0x113/0x1e0
 kthread+0x121/0x140
 ret_from_fork+0x3a/0x50

Cc: Christoph Hellwig <hch@infradead.org>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Hannes Reinecke <hare@suse.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-flush.c

@@ -69,6 +69,7 @@
 #include <linux/blkdev.h>
 #include <linux/gfp.h>
 #include <linux/blk-mq.h>
+#include <linux/lockdep.h>
 
 #include "blk.h"
 #include "blk-mq.h"
@@ -505,6 +506,9 @@ struct blk_flush_queue *blk_alloc_flush_queue(struct request_queue *q,
 	INIT_LIST_HEAD(&fq->flush_queue[1]);
 	INIT_LIST_HEAD(&fq->flush_data_in_flight);
 
+	lockdep_register_key(&fq->key);
+	lockdep_set_class(&fq->mq_flush_lock, &fq->key);
+
 	return fq;
 
  fail_rq:
@@ -519,6 +523,7 @@ void blk_free_flush_queue(struct blk_flush_queue *fq)
 	if (!fq)
 		return;
 
+	lockdep_unregister_key(&fq->key);
 	kfree(fq->flush_rq);
 	kfree(fq);
 }

block/blk.h

@@ -30,6 +30,7 @@ struct blk_flush_queue {
 	 * at the same time
 	 */
 	struct request		*orig_rq;
+	struct lock_class_key	key;
 	spinlock_t		mq_flush_lock;
 };