block: use writeback_iter

block: use writeback_iter

Use writeback_iter instead of the deprecated write_cache_pages wrapper
in blkdev_writepages.  This removes an indirect call per folio.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: John Garry <john.g.garry@oracle.com>
Link: https://lore.kernel.org/r/20250424082752.1967679-1-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>