block: mark bio_add_folio as __must_check

Now that all callers of bio_add_folio() check the return value, mark it as
__must_check.

Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/381360a45ac3684120cfbe1e07685e9c36256e47.1685532726.git.johannes.thumshirn@wdc.com


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