mm/hugetlb: make userfaultfd_huge_must_wait() safe to pmd unshare

mm/hugetlb: make userfaultfd_huge_must_wait() safe to pmd unshare

We can take the hugetlb walker lock, here taking vma lock directly.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Peter Xu <[email protected]>
Reviewed-by: David Hildenbrand <[email protected]>
Reviewed-by: Mike Kravetz <[email protected]>
Reviewed-by: John Hubbard <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: James Houghton <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Miaohe Lin <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Nadav Amit <[email protected]>
Cc: Rik van Riel <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>