vringh: use bvec_set_page to initialize a bvec

Use the bvec_set_page helper to initialize a bvec.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20230203150634.3199647-23-hch@lst.de


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