mm, hwpoison: avoid trying to unpoison reserved page

For reserved pages, HWPoison flag will be set without increasing the page
refcnt.  So we shouldn't even try to unpoison these pages and thus
decrease the page refcnt unexpectly.  Add a PageReserved() check to filter
this case out and remove the below unneeded zero page (zero page is
reserved) check.

Link: https://lkml.kernel.org/r/20220818130016.45313-7-linmiaohe@huawei.com


Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Acked-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>