selftests/vm: ksm_functional_tests: fixes for 32bit

The test currently fails on 32bit.  Fixing the "-1ull" vs.  "-1ul" seems
to make the test pass and the compiler happy.

Note: This test is not in mm-stable yet. This fix should be squashed into
      "selftests/vm: add KSM unmerge tests".

Link: https://lkml.kernel.org/r/20221205193716.276024-5-david@redhat.com


Signed-off-by: David Hildenbrand <david@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>