selftests/mm: fix build warnings on ppc64

Fix warnings like:

  In file included from uffd-unit-tests.c:8:
  uffd-unit-tests.c: In function `uffd_poison_handle_fault':
  uffd-common.h:45:33: warning: format `%llu' expects argument of type
  `long long unsigned int', but argument 3 has type `__u64' {aka `long
  unsigned int'} [-Wformat=]

By switching to unsigned long long for u64 for ppc64 builds.

Link: https://lkml.kernel.org/r/20240521030219.57439-1-mpe@ellerman.id.au


Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>