block: return -EBUSY when there are open partitions in blkdev_reread_part

block: return -EBUSY when there are open partitions in blkdev_reread_part

The switch to go through blkdev_get_by_dev means we now ignore the
return value from bdev_disk_changed in __blkdev_get.  Add a manual
check to restore the old semantics.

Fixes: 4601b4b130de ("block: reopen the device in blkdev_reread_part")
Reported-by: Karel Zak <kzak@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210421160502.447418-1-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>