block: move the code to do early boot lookup of block devices to block/

Create a new block/early-lookup.c to keep the early block device lookup
code instead of having this code sit with the early mount code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20230531125535.676098-16-hch@lst.de


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