block: initialize ret in bdev_disk_changed

Avoid a potentially initialized variabe in the invalidate case.

Fixes: d3c4a43d ("block: refactor blk_drop_partitions")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Link: https://lore.kernel.org/r/20210408194140.1816537-1-hch@lst.de


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