block: flush the integrity workqueue in blk_integrity_unregister

When the integrity profile is unregistered there can still be integrity
reads queued up which could see a NULL verify_fn as shown by the race
window below:

CPU0                                    CPU1
  process_one_work                      nvme_validate_ns
    bio_integrity_verify_fn                nvme_update_ns_info
	                                     nvme_update_disk_info
	                                       blk_integrity_unregister
                                               ---set queue->integrity as 0
	bio_integrity_process
	--access bi->profile->verify_fn(bi is a pointer of queue->integity)

Before calling blk_integrity_unregister in nvme_update_disk_info, we must
make sure that there is no work item in the kintegrityd_wq. Just call
blk_flush_integrity to flush the work queue so the bug can be resolved.

Signed-off-by: Lihong Kou <koulihong@huawei.com>
[hch: split up and shortened the changelog]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Link: https://lore.kernel.org/r/20210914070657.87677-3-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Lihong Kou

block/blk-integrity.c

@@ -430,6 +430,9 @@ void blk_integrity_unregister(struct gendisk *disk)
 
 	if (!bi->profile)
 		return;
+
+	/* ensure all bios are off the integrity workqueue */
+	blk_flush_integrity();
 	blk_queue_flag_clear(QUEUE_FLAG_STABLE_WRITES, disk->queue);
 	memset(bi, 0, sizeof(*bi));
 }