dm: open code dm_get_dev_t in dm_init_init

dm_init_init is called from early boot code, and thus lookup_bdev
will never succeed.  Just open code that call to early_lookup_bdev
instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mike Snitzer <snitzer@kernel.org>
Link: https://lore.kernel.org/r/20230531125535.676098-19-hch@lst.de


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