cgroup: kill cgrp_ss_priv[CGROUP_CANFORK_COUNT] and friends

Now that nobody use the "priv" arg passed to can_fork/cancel_fork/fork we can
kill CGROUP_CANFORK_COUNT/SUBSYS_TAG/etc and cgrp_ss_priv[] in copy_process().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
diff --git a/kernel/cgroup_pids.c b/kernel/cgroup_pids.c
index b50d5a1..18107ae 100644
--- a/kernel/cgroup_pids.c
+++ b/kernel/cgroup_pids.c
@@ -209,7 +209,7 @@
  * task_css_check(true) in pids_can_fork() and pids_cancel_fork() relies
  * on threadgroup_change_begin() held by the copy_process().
  */
-static int pids_can_fork(struct task_struct *task, void **priv_p)
+static int pids_can_fork(struct task_struct *task)
 {
 	struct cgroup_subsys_state *css;
 	struct pids_cgroup *pids;
@@ -219,7 +219,7 @@
 	return pids_try_charge(pids, 1);
 }
 
-static void pids_cancel_fork(struct task_struct *task, void *priv)
+static void pids_cancel_fork(struct task_struct *task)
 {
 	struct cgroup_subsys_state *css;
 	struct pids_cgroup *pids;