fe3cba17 added ClearPageReclaim into clear_page_dirty_for_io for
preventing fast reclaiming readahead marker page.
In this series, PG_reclaim is used by invalidated page, too.
If VM find the page is invalidated and it's dirty, it sets PG_reclaim
to reclaim asap. Then, when the dirty page will be writeback,
clear_page_dirty_for_io will clear PG_reclaim unconditionally.
It disturbs this serie's goal.
I think it's okay to clear PG_readahead when the page is dirty, not
writeback time. So this patch moves ClearPageReadahead.
This patch needs Wu's opinion.