cifs: Fix memory leak when using fscache

If we hit the 'index == next_cached' case, we leak a refcount on the
struct page.  Fix this by using readahead_folio() which takes care of
the refcount for you.

Fixes: 0174ee9 ("cifs: Implement cache I/O by accessing the cache directly")
Cc: David Howells <dhowells@redhat.com>
Cc: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/file.c

@@ -4459,18 +4459,18 @@ static void cifs_readahead(struct readahead_control *ractl)
 				 * TODO: Send a whole batch of pages to be read
 				 * by the cache.
 				 */
-				page = readahead_page(ractl);
-				last_batch_size = 1 << thp_order(page);
+				struct folio *folio = readahead_folio(ractl);
+				last_batch_size = folio_nr_pages(folio);
 				if (cifs_readpage_from_fscache(ractl->mapping->host,
-							       page) < 0) {
+							       &folio->page) < 0) {
 					/*
 					 * TODO: Deal with cache read failure
 					 * here, but for the moment, delegate
 					 * that to readpage.
 					 */
 					caching = false;
 				}
-				unlock_page(page);
+				folio_unlock(folio);
 				next_cached++;
 				cache_nr_pages--;
 				if (cache_nr_pages == 0)