bcachefs: stop using write_cache_pages

Stop using the obsolete write_cache_pages and use writeback_iter directly.
This basically just open codes write_cache_pages without the indirect
call, but there's probably ways to structure the code even nicer as a
follow on.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Christoph Hellwig <[e-mail]>
Cc: David Hildenbrand <[e-mail]>
Cc: Kent Overstreet <[e-mail]>
Cc: Konstantin Komarov <[e-mail]>
Cc: Matthew Wilcox (Oracle) <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

fs/bcachefs/fs-io-buffered.c

···
 	return 0;
 }
 
+static int bch2_write_cache_pages(struct address_space *mapping,
+		      struct writeback_control *wbc, void *data)
+{
+	struct folio *folio = NULL;
+	int error;
+
+	while ((folio = writeback_iter(mapping, wbc, folio, &error)))
+		error = __bch2_writepage(folio, wbc, data);
+	return error;
+}
+
 int bch2_writepages(struct address_space *mapping, struct writeback_control *wbc)
 {
 	struct bch_fs *c = mapping->host->i_sb->s_fs_info;
···
 	bch2_inode_opts_get(&w->opts, c, &to_bch_ei(mapping->host)->ei_inode);
 
 	blk_start_plug(&w->plug);
-	int ret = write_cache_pages(mapping, wbc, __bch2_writepage, w);
+	int ret = bch2_write_cache_pages(mapping, wbc, w);
 	if (w->io)
 		bch2_writepage_do_io(w);
 	blk_finish_plug(&w->plug);