Check if adding pages to resync bio fails and if bail out.

As the comment above suggests this cannot happen, WARN if it actually
happens. Technically __bio_add_pages() would be sufficient here, but
asserting the pages actually get added to the bio is preferred.

This way we can mark bio_add_pages as __must_check.
Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Acked-by: Song Liu <song@kernel.org>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/33aea4c271220dc9bcab58c4b7bec478c1511142.1685532726.git.johannes.thumshirn@wdc.comSigned-off-by: Jens Axboe <axboe@kernel.dk>