selftests/overlayfs: Fix build error on ppc64

Fix build error on ppc64:
  dev_in_maps.c: In function ‘get_file_dev_and_inode’:
  dev_in_maps.c:60:59: error: format ‘%llu’ expects argument of type
  ‘long long unsigned int *’, but argument 7 has type ‘__u64 *’ {aka ‘long
  unsigned int *’} [-Werror=format=]

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

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>