cgroup: make cftype->private a unsigned long

It's pretty unusual to have an int as a private data field and it
makes it impossible to carray a pointer value through it.  Let's make
it an unsigned long.  AFAICS, this shouldn't break anything.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>

include/linux/cgroup-defs.h

@@ -318,7 +318,7 @@ struct cftype {
 	 * end of cftype array.
 	 */
 	char name[MAX_CFTYPE_NAME];
-	int private;
+	unsigned long private;
 	/*
 	 * If not 0, file mode is set to this value, otherwise it will
 	 * be figured out automatically