fs: use writeback_iter directly in mpage_writepages

Stop using write_cache_pages and use writeback_iter directly.  This
removes an indirect call per written folio and makes the code easier
to follow.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/20250507062124.3933305-1-hch@lst.de


Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>