sd: factor out a sd_set_special_bvec helper

Add a helper for setting up the special_bvec instead of open coding it
in three place, and use the new bvec_set_page helper to initialize
special_vec.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Link: https://lore.kernel.org/r/20230203150634.3199647-5-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>