From: Jens Axboe <axboe@suse.de>

Dog slow software suspend found this one. If WB_SYNC_ALL, then you need
to mark the bio as sync as well.

This is because swap_writepage() does a remove_exclusive_swap_page() (going
to __delete_from_swap_cache -> __remove_from_page_cache) which can kill
page->mapping, thus aops->sync_page() has nothing to work with for unplugging
the address space.