zram: use __bio_add_page for adding single page to bio

The zram writeback code uses bio_add_page() to add a page to a newly
created bio. bio_add_page() can fail, but the return value is never
checked.

Use __bio_add_page() as adding a single page to a newly created bio is
guaranteed to succeed.

This brings us a step closer to marking bio_add_page() as __must_check.
Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/cfd141dd7773315879a126f2aa81b7f698bc0e10.1685532726.git.johannes.thumshirn@wdc.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/zram/zram_drv.c

@@ -700,7 +700,7 @@ static ssize_t writeback_store(struct device *dev,
 		bio_init(&bio, zram->bdev, &bio_vec, 1,
 			 REQ_OP_WRITE | REQ_SYNC);
 		bio.bi_iter.bi_sector = blk_idx * (PAGE_SIZE >> 9);
-		bio_add_page(&bio, page, PAGE_SIZE, 0);
+		__bio_add_page(&bio, page, PAGE_SIZE, 0);
 
 		/*
 		 * XXX: A single page IO would be inefficient for write