fs: Convert __page_get_link() to use a folio

Retrieve a folio from the page cache instead of a page and operate
on it. Removes two hidden calls to compound_head().

Signed-off-by: "Matthew Wilcox (Oracle)" <[e-mail]>
Link: https://lore.kernel.org/[e-mail]
Signed-off-by: Christian Brauner <[e-mail]>

fs/namei.c

···
 static char *__page_get_link(struct dentry *dentry, struct inode *inode,
 			     struct delayed_call *callback)
 {
-	struct page *page;
+	struct folio *folio;
 	struct address_space *mapping = inode->i_mapping;
 
 	if (!dentry) {
-		page = find_get_page(mapping, 0);
-		if (!page)
+		folio = filemap_get_folio(mapping, 0);
+		if (IS_ERR(folio))
 			return ERR_PTR(-ECHILD);
-		if (!PageUptodate(page)) {
-			put_page(page);
+		if (!folio_test_uptodate(folio)) {
+			folio_put(folio);
 			return ERR_PTR(-ECHILD);
 		}
 	} else {
-		page = read_mapping_page(mapping, 0, NULL);
-		if (IS_ERR(page))
-			return (char*)page;
+		folio = read_mapping_folio(mapping, 0, NULL);
+		if (IS_ERR(folio))
+			return ERR_CAST(folio);
 	}
-	set_delayed_call(callback, page_put_link, page);
+	set_delayed_call(callback, page_put_link, &folio->page);
 	BUG_ON(mapping_gfp_mask(mapping) & __GFP_HIGHMEM);
-	return page_address(page);
+	return folio_address(folio);
 }
 
 const char *page_get_link_raw(struct dentry *dentry, struct inode *inode,