fs/proc/task_mmu: properly detect PM_MMAP_EXCLUSIVE per page of PMD-mapped THPs

We added PM_MMAP_EXCLUSIVE in 2015 via commit 77bb499bb60f ("pagemap: add
mmap-exclusive bit for marking pages mapped only here"), when THPs could
not be partially mapped and page_mapcount() returned something that was
true for all pages of the THP.

In 2016, we added support for partially mapping THPs via commit
53f9263baba6 ("mm: rework mapcount accounting to enable 4k mapping of
THPs") but missed to determine PM_MMAP_EXCLUSIVE as well per page.

Checking page_mapcount() on the head page does not tell the whole story.

We should check each individual page.  In a future without per-page
mapcounts it will be different, but we'll change that to be consistent
with PTE-mapped THPs once we deal with that.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 53f9263baba6 ("mm: rework mapcount accounting to enable 4k mapping of THPs")
Signed-off-by: David Hildenbrand <[email protected]>
Reviewed-by: Oscar Salvador <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Alexey Dobriyan <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Lance Yang <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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