block: fix busy device checking in blk_drop_partitions again

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>

block/partitions/core.c

@@ -496,7 +496,7 @@ int blk_drop_partitions(struct gendisk *disk, struct block_device *bdev)
 
 	if (!disk_part_scan_enabled(disk))
 		return 0;
-	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)