mm/memory_hotplug: use pgprot_val to get value of pgprot

pgprot.pgprot is non-portable code.  It should be replaced by portable
macro pgprot_val.

Link: https://lkml.kernel.org/r/20220426071302.220646-1-liusongtang@huawei.com


Signed-off-by: liusongtang <liusongtang@huawei.com>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Xiaoming Ni <nixiaoming@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>