mm: Use ptep_get() for accessing PTE entries

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

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-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>