block: fix busy device checking in blk_drop_partitions

mainline inclusion
from mainline-v5.7-rc1
commit d3ef5536
category: bugfix
bugzilla: 186898, https://gitee.com/openeuler/kernel/issues/I5ALFU
CVE: NA

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

bd_super is only set by get_tree_bdev and mount_bdev, and thus not by
other openers like btrfs or the XFS realtime and log devices, as well as
block devices directly opened from user space.  Check bd_openers
instead.

Fixes: 77032ca6 ("Return EBUSY from BLKRRPART for mounted whole-dev fs")
Signed-off-by: NChristoph Hellwig <hch@lst.de>
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_super)
+	if (bdev->bd_part_count || bdev->bd_openers)
 		return -EBUSY;
 	res = invalidate_partition(disk, 0);
 	if (res)