init: factor the root_wait logic in prepare_namespace into a helper

The root_wait logic is a bit obsfucated right now.  Expand it and move it
into a helper.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20230531125535.676098-12-hch@lst.de


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