hugetlb: do not use vma_hugecache_offset() for vma_prio_tree_foreach

hugetlb: do not use vma_hugecache_offset() for vma_prio_tree_foreach

0c176d5 ("mm: hugetlb: fix pgoff computation when unmapping page from
vma") fixed pgoff calculation but it has replaced it by
vma_hugecache_offset() which is not approapriate for offsets used for
vma_prio_tree_foreach() because that one expects index in page units
rather than in huge_page_shift.

Using vma_hugecache_offset() is not incorrect because the pgoff will fit
into the same vmas but it is confusing so the standard PAGE_SHIFT based
index calculation is used instead.

Signed-off-by: Michal Hocko <mhocko@suse.cz>
Acked-by: Hillf Danton <dhillf@gmail.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>