arm64: mm: implement the architecture-specific test_and_clear_young_ptes()

Implement the Arm64 architecture-specific test_and_clear_young_ptes() to
enable batched checking of young flags, improving performance during large
folio reclamation when MGLRU is enabled.

While we're at it, simplify ptep_test_and_clear_young() by calling
test_and_clear_young_ptes(). Since callers guarantee that PTEs are
present before calling these functions, we can use pte_cont() to check the
CONT_PTE flag instead of pte_valid_cont().

Performance testing:

Enable MGLRU, then allocate 10G clean file-backed folios by mmap() in a
memory cgroup, and try to reclaim 8G file-backed folios via the
memory.reclaim interface. I can observe 60%+ performance improvement on
my Arm64 32-core server (and about 15% improvement on my X86 machine).

W/o patchset:
real 0m0.470s
user 0m0.000s
sys 0m0.470s

W/ patchset:
real 0m0.180s
user 0m0.001s
sys 0m0.179s

Link: https://lkml.kernel.org/r/7f891d42a720cc2e57862f3b79e4f774404f313c.1772778858.git.baolin.wang@linux.alibaba.com
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Rik van Riel <riel@surriel.com>
Reviewed-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Jann Horn <jannh@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Wei Xu <weixugc@google.com>
Cc: Will Deacon <will@kernel.org>
Cc: Yuanchu Xie <yuanchu@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>