selftests/cgroup: remove ARRAY_SIZE define from cgroup_util.h

ARRAY_SIZE is defined in several selftests. Remove definitions from
individual test files and include header file for the define instead.
ARRAY_SIZE define is added in a separate patch to prepare for this
change.

Remove ARRAY_SIZE from cgroup_util.h and pickup the one defined in
kselftest.h.
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/cgroup/cgroup_util.h

@@ -2,9 +2,9 @@
 #include <stdbool.h>
 #include <stdlib.h>
 
-#define PAGE_SIZE 4096
+#include "../kselftest.h"
 
-#define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]))
+#define PAGE_SIZE 4096
 
 #define MB(x) (x << 20)