block: fix busy device checking in blk_drop_partitions again

mainline inclusion
from mainline-v5.7-rc1
commit cb6b771b
category: bugfix
bugzilla: 186898, https://gitee.com/openeuler/kernel/issues/I5ALJL
CVE: NA

--------------------------------

The previous fix had an off by one in the bd_openers checking, counting
the callers blkdev_get.

Fixes: d3ef5536 ("block: fix busy device checking in blk_drop_partitions")
Reported-by: NQian Cai <cai@lca.pw>
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Tested-by: NQian Cai <cai@lca.pw>
Signed-off-by: NJens Axboe <axboe@kernel.dk>
Signed-off-by: NLi Nan <linan122@huawei.com>
Reviewed-by: NJason Yan <yanaijie@huawei.com>
Signed-off-by: NYongqiang Liu <liuyongqiang13@huawei.com>

block/partition-generic.c

@@ -464,7 +464,7 @@ static int drop_partitions(struct gendisk *disk, struct block_device *bdev)
 	struct hd_struct *part;
 	int res;
 
-	if (bdev->bd_part_count || bdev->bd_openers)
+	if (bdev->bd_part_count || bdev->bd_openers > 1)
 		return -EBUSY;
 	res = invalidate_partition(disk, 0);
 	if (res)