init: refactor mount_root

Provide stubs for all the lower level mount helpers, and just switch
on ROOT_DEV in the main function.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20230531125535.676098-8-hch@lst.de


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