Revert "arm64: hugetlb: partial revert of 66b3923a"

This reverts commit ff792584.

Now that the contiguous-hint hugetlb regression has been debugged and
fixed upstream by 66ee95d1

 ("mm: exclude HugeTLB pages from THP
page_mapped() logic"), we can revert the previous partial revert of this
feature.

Signed-off-by: Will Deacon <will.deacon@arm.com>