We have to update the uffd-wp SWP PTE bit independent of the type of
migration entry.  Currently, if we're unlucky and we want to install/clear
the uffd-wp bit just while we're migrating a read-only mapped hugetlb
page, we would miss to set/clear the uffd-wp bit.

Further, if we're processing a readable-exclusive migration entry and
neither want to set or clear the uffd-wp bit, we could currently end up
losing the uffd-wp bit.  Note that the same would hold for writable
migrating entries, however, having a writable migration entry with the
uffd-wp bit set would already mean that something went wrong.

Note that the change from !is_readable_migration_entry ->
writable_migration_entry is harmless and actually cleaner, as raised by
Miaohe Lin and discussed in [1].

[1] https://lkml.kernel.org/r/90dd6a93-4500-e0de-2bf0-bf522c311b0c@huawei.com

Link: https://lkml.kernel.org/r/20221222205511.675832-3-david@redhat.com
Fixes: 60dfaad6 ("mm/hugetlb: allow uffd wr-protect none ptes")
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Peter Xu <peterx@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>