selftests/mm: test UFFDIO_ZEROPAGE only when !hugetlb

Make the check as simple as "test_type == TEST_HUGETLB" because that's the
only mem that doesn't support ZEROPAGE.

Link: https://lkml.kernel.org/r/20230412164234.328168-1-peterx@redhat.com


Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: 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>