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: Qian Cai <cai@lca.pw>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Qian Cai <cai@lca.pw>
Signed-off-by: Jens Axboe <axboe@kernel.dk>