f2fs: convert f2fs_set_compressed_page() to use folio

Convert to use folio, so that we can get rid of 'page->index' to
prepare for removal of 'index' field in structure page [1].

[1] https://lore.kernel.org/all/Zp8fgUSIBGQ1TN0D@casper.infradead.org/

Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Chao Yu <chao@kernel.org>
Reviewed-by: Li Zetao <lizetao1@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/compress.c

@@ -90,11 +90,13 @@ bool f2fs_is_compressed_page(struct page *page)
 static void f2fs_set_compressed_page(struct page *page,
 		struct inode *inode, pgoff_t index, void *data)
 {
-	attach_page_private(page, (void *)data);
+	struct folio *folio = page_folio(page);
+
+	folio_attach_private(folio, (void *)data);
 
 	/* i_crypto_info and iv index */
-	page->index = index;
-	page->mapping = inode->i_mapping;
+	folio->index = index;
+	folio->mapping = inode->i_mapping;
 }
 
 static void f2fs_drop_rpages(struct compress_ctx *cc, int len, bool unlock)