init: move the nfs/cifs/ram special cases out of name_to_dev_t

The nfs/cifs/ram special case only needs to be parsed once, and only in
the boot code.  Move them out of name_to_dev_t and into
prepare_namespace.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20230531125535.676098-13-hch@lst.de


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