block: factor out a bvec_set_page helper

Add a helper to initialize a bvec based of a page pointer.  This will help
removing various open code bvec initializations.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/20230203150634.3199647-2-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>