md: use __register_blkdev to allocate devices on demand

Use the simpler mechanism attached to major_name to allocate a md device
when a currently unregistered minor is accessed.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Acked-by: Song Liu <song@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>