selftests/cgroup: Fix build on older distros

On older distros struct clone_args does not have a cgroup member,
leading to build errors:

 cgroup_util.c: In function 'clone_into_cgroup':
 cgroup_util.c:343:4: error: 'struct clone_args' has no member named 'cgroup'
 cgroup_util.c:346:33: error: invalid application of 'sizeof' to incomplete
  type 'struct clone_args'

But the selftests already have a locally defined version of the
structure which is up to date, called __clone_args.

So use __clone_args which fixes the error.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Sachin Sant <sachinp@linux.vnet.ibm.com&gt;>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>