fs/proc/task_mmu: don't indicate PM_MMAP_EXCLUSIVE without PM_PRESENT

Relying on the mapcount for non-present PTEs that reference pages doesn't
make any sense: they are not accounted in the mapcount, so page_mapcount()
== 1 won't return the result we actually want to know.

While we don't check the mapcount for migration entries already, we could
end up checking it for swap, hwpoison, device exclusive, ...  entries,
which we really shouldn't.

There is one exception: device private entries, which we consider
fake-present (e.g., incremented the mapcount).  But we won't care about
that for now for PM_MMAP_EXCLUSIVE, because indicating PM_SWAP for them
although they are fake-present already sounds suspiciously wrong.

Let's never indicate PM_MMAP_EXCLUSIVE without PM_PRESENT.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: David Hildenbrand <[email protected]>
Reviewed-by: Oscar Salvador <[email protected]>
Cc: Alexey Dobriyan <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Lance Yang <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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