zram: use bvec_set_page to initialize bvecs

Use the bvec_set_page helper to initialize bvecs.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/20230203150634.3199647-11-hch@lst.de


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