dasd: use bdev_disk_changed instead of blk_drop_partitions

Use the more general interface - the behavior is the same except
that now a change uevent is sent, which is the right thing to do
when the device becomes unusable.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Stefan Haberland <sth@linux.ibm.com>
Link: https://lore.kernel.org/r/20210406062303.811835-2-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>