readahead: Convert page_cache_async_ra() to take a folio

Using the folio here avoids checking whether it's a tail page.
This patch mostly just enables some of the following patches.
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>

include/linux/pagemap.h

@@ -993,7 +993,7 @@ struct readahead_control {
 void page_cache_ra_unbounded(struct readahead_control *,
 		unsigned long nr_to_read, unsigned long lookahead_count);
 void page_cache_sync_ra(struct readahead_control *, unsigned long req_count);
-void page_cache_async_ra(struct readahead_control *, struct page *,
+void page_cache_async_ra(struct readahead_control *, struct folio *,
 		unsigned long req_count);
 void readahead_expand(struct readahead_control *ractl,
 		      loff_t new_start, size_t new_len);
@@ -1040,7 +1040,7 @@ void page_cache_async_readahead(struct address_space *mapping,
 		struct page *page, pgoff_t index, unsigned long req_count)
 {
 	DEFINE_READAHEAD(ractl, file, ra, mapping, index);
-	page_cache_async_ra(&ractl, page, req_count);
+	page_cache_async_ra(&ractl, page_folio(page), req_count);
 }
 
 static inline struct folio *__readahead_folio(struct readahead_control *ractl)

mm/readahead.c

@@ -581,7 +581,7 @@ void page_cache_sync_ra(struct readahead_control *ractl,
 EXPORT_SYMBOL_GPL(page_cache_sync_ra);
 
 void page_cache_async_ra(struct readahead_control *ractl,
-		struct page *page, unsigned long req_count)
+		struct folio *folio, unsigned long req_count)
 {
 	/* no read-ahead */
 	if (!ractl->ra->ra_pages)
@@ -590,10 +590,10 @@ void page_cache_async_ra(struct readahead_control *ractl,
 	/*
 	 * Same bit is used for PG_readahead and PG_reclaim.
 	 */
-	if (PageWriteback(page))
+	if (folio_test_writeback(folio))
 		return;
 
-	ClearPageReadahead(page);
+	folio_clear_readahead(folio);
 
 	/*
 	 * Defer asynchronous read-ahead on IO congestion.