mm: Use pmdp_get() for accessing PMD entries

Convert PMD accesses via pmdp_get() helper that defaults as READ_ONCE() but
also provides the platform an opportunity to override when required.

Cc: Dimitri Sivanich <dimitri.sivanich@hpe.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Naoya Horiguchi <nao.horiguchi@gmail.com>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: Dennis Zhou <dennis@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Uladzislau Rezki <urezki@gmail.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: "Mike Rapoport (IBM)" <rppt@kernel.org>
Cc: linux-kernel@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: kasan-dev@googlegroups.com
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>