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: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Li Nan <linan122@huawei.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>