mm: Use pgdp_get() for accessing PGD entries

Convert PGD accesses via pgdp_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: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Miaohe Lin <linmiaohe@huawei.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: linux-kernel@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: linux-perf-users@vger.kernel.org
Cc: kasan-dev@googlegroups.com
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>