selftests/mm: fix split huge page tests

Fix two inputs to check_anon_huge() and one if condition, so the tests
work as expected.

Link: https://lkml.kernel.org/r/20230306160907.16804-1-zi.yan@sent.com
Fixes: c07c343c

 ("selftests/vm: dedup THP helpers")
Signed-off-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Zach O'Keefe <zokeefe@google.com>
Tested-by: Zach O'Keefe <zokeefe@google.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>