block: mark bio_add_page as __must_check

Now that all users of bio_add_page check for the return value, mark
bio_add_page as __must_check.

Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/7ae4a902e08fe2e90c012ee07aeb35d4aae28373.1685532726.git.johannes.thumshirn@wdc.com


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