block: add a bvec_set_virt helper

A small wrapper around bvec_set_page for callers that have a virtual
address.

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/20230203150634.3199647-4-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>