zram: use __bio_add_page for adding single page to bio

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