fs: nfs: fix missing refcnt by replacing folio_set_private by folio_attach_private

fs: nfs: fix missing refcnt by replacing folio_set_private by folio_attach_private

This patch is inspired by a code review of fs codes which aims at
folio's extra refcnt that could introduce unwanted behavious when
judging refcnt, such as[1].That is, the folio passed to
mapping_evict_folio carries the refcnts from find_lock_entries,
page_cache, corresponding to PTEs and folio's private if has. However,
current code doesn't take the refcnt for folio's private which could
have mapping_evict_folio miss the one to only PTE and lead to
call filemap_release_folio wrongly.

[1]
long mapping_evict_folio(struct address_space *mapping, struct folio *folio)
{
...
//current code will misjudge here if there is one pte on the folio which
is be deemed as the one as folio's private
        if (folio_ref_count(folio) >
                        folio_nr_pages(folio) + folio_has_private(folio) + 1)
                return 0;
        if (!filemap_release_folio(folio, 0))
                return 0;

        return remove_mapping(mapping, folio);
}

Signed-off-by: Zhaoyang Huang <zhaoyang.huang@unisoc.com>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>