arm64/hugetlb: Implement arm64 specific huge_ptep_get()

Now we use huge_ptep_get() to get the pte value of a hugetlb page,
however it will only return one specific pte value for the CONT-PTE
or CONT-PMD size hugetlb on ARM64 system, which can contain several
continuous pte or pmd entries with same page table attributes. And it
will not take into account the subpages' dirty or young bits of a
CONT-PTE/PMD size hugetlb page.

So the huge_ptep_get() is inconsistent with huge_ptep_get_and_clear(),
which already takes account the dirty or young bits for any subpages
in this CONT-PTE/PMD size hugetlb [1]. Meanwhile we can miss dirty or
young flags statistics for hugetlb pages with current huge_ptep_get(),
such as the gather_hugetlb_stats() function, and CONT-PTE/PMD hugetlb
monitoring with DAMON.

Thus define an ARM64 specific huge_ptep_get() implementation as well as
enabling __HAVE_ARCH_HUGE_PTEP_GET, that will take into account any
subpages' dirty or young bits for CONT-PTE/PMD size hugetlb page, for
those functions that want to check the dirty and young flags of a hugetlb
page.

[1] https://lore.kernel.org/linux-mm/85bd80b4-b4fd-0d3f-a2e5-149559f2f387@oracle.com/



Suggested-by: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Link: https://lore.kernel.org/r/624109a80ac4bbdf1e462dfa0b49e9f7c31a7c0d.1652496622.git.baolin.wang@linux.alibaba.com


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>