selftests/mm: add shmem-private test to uffd-stress

The userfaultfd stress test never tested private shmem, which I think was
overlooked long due.  Add it so it matches with uffd unit test and it'll
cover all memory supported with the three memory types.

Meanwhile, rename the memory types a bit.  Considering shared mem is the
major use case for both shmem / hugetlbfs, changing from:

  anon, hugetlb, hugetlb_shared, shmem

To (with shmem-private added):

  anon, hugetlb, hugetlb-private, shmem, shmem-private

Add the shmem-private to run_vmtests.sh too.

Link: https://lkml.kernel.org/r/20230412164546.329355-1-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Dmitry Safonov <0x7f454c46@gmail.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Zach O'Keefe <zokeefe@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>