mm: rmap: rename page_not_mapped() to folio_not_mapped()

Since commit 2f031c6f ("mm/rmap: Convert rmap_walk() to take a
folio"), page_not_mapped() takes folio as parameter, rename it to be
consistent.

Link: https://lkml.kernel.org/r/20220927063826.159590-1-wangkefeng.wang@huawei.com


Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>