fs: buffer: use __bio_add_page to add single page to bio

fs: buffer: use __bio_add_page to add single page to bio

The buffer_head submission 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: Gou Hao <gouhao@uniontech.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/84ff2dcbe81b258a73ad900adb5266e208b61a4d.1685532726.git.johannes.thumshirn@wdc.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>