mm/hugetlb: separate path for hwpoison entry in copy_hugetlb_page_range()

Originally copy_hugetlb_page_range() handles migration entries and
hwpoisoned entries in similar manner.  But recently the related code path
has more code for migration entries, and when
is_writable_migration_entry() was converted to
!is_readable_migration_entry(), hwpoison entries on source processes got
to be unexpectedly updated (which is legitimate for migration entries, but
not for hwpoison entries).  This results in unexpected serious issues like
kernel panic when forking processes with hwpoison entries in pmd.

Separate the if branch into one for hwpoison entries and one for migration
entries.

Link: https://lkml.kernel.org/r/20220704013312.2415700-3-naoya.horiguchi@linux.dev
Fixes: 6c287605

 ("mm: remember exclusively mapped anonymous pages with PG_anon_exclusive")
Signed-off-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Cc: <stable@vger.kernel.org>	[5.18]
Cc: David Hildenbrand <david@redhat.com>
Cc: Liu Shixin <liushixin2@huawei.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Yang Shi <shy828301@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>