ext4: Use a folio in ext4_read_merkle_tree_page

This is an implementation of fsverity_operations read_merkle_tree_page,
so it must still return the precise page asked for, but we can use the
folio API to reduce the number of conversions between folios & pages.
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://lore.kernel.org/r/20230324180129.1220691-30-willy@infradead.orgSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/verity.c

@@ -361,21 +361,21 @@ static struct page *ext4_read_merkle_tree_page(struct inode *inode,
 					       pgoff_t index,
 					       unsigned long num_ra_pages)
 {
-	struct page *page;
+	struct folio *folio;
 
 	index += ext4_verity_metadata_pos(inode) >> PAGE_SHIFT;
 
-	page = find_get_page_flags(inode->i_mapping, index, FGP_ACCESSED);
-	if (!page || !PageUptodate(page)) {
+	folio = __filemap_get_folio(inode->i_mapping, index, FGP_ACCESSED, 0);
+	if (!folio || !folio_test_uptodate(folio)) {
 		DEFINE_READAHEAD(ractl, NULL, NULL, inode->i_mapping, index);
 
-		if (page)
-			put_page(page);
+		if (folio)
+			folio_put(folio);
 		else if (num_ra_pages > 1)
 			page_cache_ra_unbounded(&ractl, num_ra_pages, 0);
-		page = read_mapping_page(inode->i_mapping, index, NULL);
+		folio = read_mapping_folio(inode->i_mapping, index, NULL);
 	}
-	return page;
+	return folio_file_page(folio, index);
 }
 
 static int ext4_write_merkle_tree_block(struct inode *inode, const void *buf,