mm/huge_memory: check pmd_special() only after pmd_present()

We should only check for pmd_special() after we made sure that we have a
present PMD.  For example, if we have a migration PMD, pmd_special() might
indicate that we have a special PMD although we really don't.

This fixes confusing migration entries as PFN mappings, and not doing what
we are supposed to do in the "is_swap_pmd()" case further down in the
function -- including messing up COW, page table handling and accounting.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: bc02afbd4d73 ("mm/fork: accept huge pfnmap entries")
Signed-off-by: David Hildenbrand <[email protected]>
Reported-by: [email protected]
Closes: https://lore.kernel.org/lkml/[email protected]/
Reviewed-by: Peter Xu <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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