mm/page_alloc: use folio_mapped() in __alloc_contig_migrate_range()

We want to limit the use of page_mapcount() to the places where it is
absolutely necessary.

For tracing purposes, we use page_mapcount() in
__alloc_contig_migrate_range().  Adding that mapcount to total_mapped
sounds strange: total_migrated and total_reclaimed would count each page
only once, not multiple times.

But then, isolate_migratepages_range() adds each folio only once to the
list.  So for large folios, we would query the mapcount of the first page
of the folio, which doesn't make too much sense for large folios.

Let's simply use folio_mapped() * folio_nr_pages(), which makes more sense
as nr_migratepages is also incremented by the number of pages in the folio
in case of successful migration.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: David Hildenbrand <[email protected]>
Cc: Chris Zankel <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: John Paul Adrian Glaubitz <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Max Filippov <[email protected]>
Cc: Miaohe Lin <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Richard Chang <[email protected]>
Cc: Rich Felker <[email protected]>
Cc: Ryan Roberts <[email protected]>
Cc: Yang Shi <[email protected]>
Cc: Yin Fengwei <[email protected]>
Cc: Yoshinori Sato <[email protected]>
Cc: Zi Yan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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