virtio_blk: use bvec_set_virt to initialize special_vec

Use the bvec_set_virt helper to initialize the special_vec.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20230203150634.3199647-10-hch@lst.de


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