init: pass root_device_name explicitly

Instead of declaring root_device_name as a global variable pass it as an
argument to the functions using it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20230531125535.676098-9-hch@lst.de


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