mm/page-writeback: drop usage of folio_index

mm/page-writeback: drop usage of folio_index

folio_index is only needed for mixed usage of page cache and swap cache.
The remaining three caller in page-writeback are for page cache tag
marking.  Swap cache space doesn't use tag (explicitly sets
mapping_set_no_writeback_tags), so use folio->index here directly.

Link: https://lkml.kernel.org/r/20250825163721.17734-1-ryncsn@gmail.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>