mips: use nth_page() in place of direct struct page manipulation

__flush_dcache_pages() is called during hugetlb migration via
migrate_pages() -> migrate_hugetlbs() -> unmap_and_move_huge_page() ->
move_to_new_folio() -> flush_dcache_folio().  And with hugetlb and without
sparsemem vmemmap, struct page is not guaranteed to be contiguous beyond a
section.  Use nth_page() instead.

Without the fix, a wrong address might be used for data cache page flush.
No bug is reported. The fix comes from code inspection.

Link: https://lkml.kernel.org/r/20230913201248.452081-6-zi.yan@sent.com
Fixes: 15fa3e8e ("mips: implement the new page table range API")
Signed-off-by: Zi Yan <ziy@nvidia.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

arch/mips/mm/cache.c

@@ -117,7 +117,7 @@ void __flush_dcache_pages(struct page *page, unsigned int nr)
 	 * get faulted into the tlb (and thus flushed) anyways.
 	 */
 	for (i = 0; i < nr; i++) {
-		addr = (unsigned long)kmap_local_page(page + i);
+		addr = (unsigned long)kmap_local_page(nth_page(page, i));
 		flush_data_cache_page(addr);
 		kunmap_local((void *)addr);
 	}