selftests/mm: drop global hpage_size in uffd tests

hpage_size was wrongly used.  Sometimes it means hugetlb default size,
sometimes it was used as thp size.

Remove the global variable and use the right one at each place.

Link: https://lkml.kernel.org/r/20230412164333.328596-1-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Mike Rapoport (IBM) <rppt@kernel.org>
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: Zach O'Keefe <zokeefe@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>