mm/hugetlb: make pud_huge() and follow_huge_pud() aware of non-present pud entry

mm/hugetlb: make pud_huge() and follow_huge_pud() aware of non-present pud entry

follow_pud_mask() does not support non-present pud entry now.  As long as
I tested on x86_64 server, follow_pud_mask() still simply returns
no_page_table() for non-present_pud_entry() due to pud_bad(), so no severe
user-visible effect should happen.  But generally we should call
follow_huge_pud() for non-present pud entry for 1GB hugetlb page.

Update pud_huge() and follow_huge_pud() to handle non-present pud entries.
The changes are similar to previous works for pud entries commit
e66f17ff7177 ("mm/hugetlb: take page table lock in follow_huge_pmd()") and
commit cbef8478bee5 ("mm/hugetlb: pmd_huge() returns true for non-present
hugepage").

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Naoya Horiguchi <[email protected]>
Reviewed-by: Miaohe Lin <[email protected]>
Reviewed-by: Mike Kravetz <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: kernel test robot <[email protected]>
Cc: Liu Shixin <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Oscar Salvador <[email protected]>
Cc: Yang Shi <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>