mm/shmem: fix shmem folio swapoff hang

Shmem swapoff makes no progress: the index to indices is not incremented. 
But "ret" is no longer a return value, so use folio_batch_count() instead.

Link: https://lkml.kernel.org/r/c32bee8a-f0aa-245-f94e-24dd271924fa@google.com
Fixes: da08e9b ("mm/shmem: convert shmem_swapin_page() to shmem_swapin_folio()")
Signed-off-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>
Tested-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Hugh Dickins

mm/shmem.c

@@ -1164,7 +1164,6 @@ static int shmem_find_swap_entries(struct address_space *mapping,
 	XA_STATE(xas, &mapping->i_pages, start);
 	struct folio *folio;
 	swp_entry_t entry;
-	unsigned int ret = 0;
 
 	rcu_read_lock();
 	xas_for_each(&xas, folio, ULONG_MAX) {
@@ -1178,7 +1177,7 @@ static int shmem_find_swap_entries(struct address_space *mapping,
 		if (swp_type(entry) != type)
 			continue;
 
-		indices[ret] = xas.xa_index;
+		indices[folio_batch_count(fbatch)] = xas.xa_index;
 		if (!folio_batch_add(fbatch, folio))
 			break;