block: allow passing a NULL bdev to bio_alloc_clone/bio_init_clone

Device mapper wants to allocate a bio before knowing the device it
gets send to, so add explicit support for that.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mike Snitzer <snitzer@kernel.org>
Link: https://lore.kernel.org/r/20220504142950.567582-3-hch@lst.de


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