ext4: switch to using write_cache_pages() for data=journal writeout

Instead of using generic_writepages(), let's use write_cache_pages() for
writeout of journalled data. It will allow us to stop providing
.writepage callback. Our data=journal writeback path would benefit from
a larger cleanup and refactoring but that's for a separate cleanup
series.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20221207112722.22220-10-jack@suse.cz
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/inode.c

@@ -2712,6 +2712,12 @@ static int mpage_prepare_extent_to_map(struct mpage_da_data *mpd)
 	return err;
 }
 
+static int ext4_writepage_cb(struct page *page, struct writeback_control *wbc,
+			     void *data)
+{
+	return ext4_writepage(page, wbc);
+}
+
 static int ext4_do_writepages(struct mpage_da_data *mpd)
 {
 	struct writeback_control *wbc = mpd->wbc;
@@ -2738,7 +2744,9 @@ static int ext4_do_writepages(struct mpage_da_data *mpd)
 		goto out_writepages;
 
 	if (ext4_should_journal_data(inode)) {
-		ret = generic_writepages(mapping, wbc);
+		blk_start_plug(&plug);
+		ret = write_cache_pages(mapping, wbc, ext4_writepage_cb, NULL);
+		blk_finish_plug(&plug);
 		goto out_writepages;
 	}