dm-snap: simplify the origin_dev == cow_dev check in snapshot_ctr

Use the block_device acquired in dm_get_device for the check instead
of doing an extra lookup.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mike Snitzer <snitzer@kernel.org>
Link: https://lore.kernel.org/r/20230531125535.676098-18-hch@lst.de


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