mm: vmscan: do not use page_count without a page pin

mm: vmscan: do not use page_count without a page pin

It is unsafe to run page_count during the physical pfn scan because
compound_head could trip on a dangling pointer when reading
page->first_page if the compound page is being freed by another CPU.

[[email protected]: split out patch]
Signed-off-by: Andrea Arcangeli <[email protected]>
Signed-off-by: Mel Gorman <[email protected]>
Reviewed-by: Michal Hocko <[email protected]>
Reviewed-by: Minchan Kim <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>