block: add a bvec_set_folio helper

A smaller wrapper around bvec_set_page that takes a folio instead.
There are only two potential users for this in the tree, but the number
will grow in the future.

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/20230203150634.3199647-3-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>