mmc: block: cleanup gendisk creation

Restructure mmc_blk_probe to avoid a failure path with a half created
disk.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20210809064028.1198327-3-hch@lst.de


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