fs: Convert simple_readpage to simple_read_folio

This is a full folio conversion; it is prepared to handle folios of
arbitrary size.
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

fs/libfs.c

@@ -539,12 +539,12 @@ int simple_setattr(struct user_namespace *mnt_userns, struct dentry *dentry,
 }
 EXPORT_SYMBOL(simple_setattr);
 
-static int simple_readpage(struct file *file, struct page *page)
+static int simple_read_folio(struct file *file, struct folio *folio)
 {
-	clear_highpage(page);
-	flush_dcache_page(page);
-	SetPageUptodate(page);
-	unlock_page(page);
+	folio_zero_range(folio, 0, folio_size(folio));
+	flush_dcache_folio(folio);
+	folio_mark_uptodate(folio);
+	folio_unlock(folio);
 	return 0;
 }
 
@@ -592,7 +592,7 @@ EXPORT_SYMBOL(simple_write_begin);
  * should extend on what's done here with a call to mark_inode_dirty() in the
  * case that i_size has changed.
  *
- * Use *ONLY* with simple_readpage()
+ * Use *ONLY* with simple_read_folio()
  */
 static int simple_write_end(struct file *file, struct address_space *mapping,
 			loff_t pos, unsigned len, unsigned copied,
@@ -628,7 +628,7 @@ static int simple_write_end(struct file *file, struct address_space *mapping,
  * Provides ramfs-style behavior: data in the pagecache, but no writeback.
  */
 const struct address_space_operations ram_aops = {
-	.readpage	= simple_readpage,
+	.read_folio	= simple_read_folio,
 	.write_begin	= simple_write_begin,
 	.write_end	= simple_write_end,
 	.dirty_folio	= noop_dirty_folio,