mtd: block2mtd: factor the early block device open logic into a helper

Simplify add_device a bit by splitting out the cumbersome early boot logic
into a separate helper.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20230531125535.676098-23-hch@lst.de


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