mm/uffd: sanity check write bit for uffd-wp protected ptes

Let's add one sanity check for CONFIG_DEBUG_VM on the write bit in
whatever chance we have when walking through the pgtables.  It can bring
the error earlier even before the app notices the data was corrupted on
the snapshot.  Also it helps us to identify this is a wrong pgtable setup,
so hopefully a great information to have for debugging too.

Link: https://lkml.kernel.org/r/20221114000447.1681003-3-peterx@redhat.com


Signed-off-by: Peter Xu <peterx@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>