block: WARN_ON_ONCE() when we remove active partitions

block: WARN_ON_ONCE() when we remove active partitions

The logic for disk->open_partitions is:

blkdev_get_by_*()
-> bdev_is_partition()
   -> blkdev_get_part()
      -> blkdev_get_whole() // bdev_whole->bd_openers++
      -> if (part->bd_openers == 0)
                 disk->open_partitions++
         part->bd_openers

In other words, when we first claim/open a partition we increment
disk->open_partitions and only when all part->bd_openers are closed will
disk->open_partitions be zero. That should mean that
disk->open_partitions is always > 0 as long as there's anyone that
has an open partition.

So the check for disk->open_partitions should mean that we can never
remove an active partition that has a holder and holder ops set. Assert
that in the code. The main disk isn't removed so that check doesn't work
for disk->part0 which is what we want. After all we only care about
partition not about the main disk.

Link: https://lore.kernel.org/r/20231017184823.1383356-3-hch@lst.de
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Christian Brauner <brauner@kernel.org>