rcu: Make task_subsys_state() RCU-lockdep checks handle boot-time use

It is apparently legal to invoke task_subsys_state() without RCU
protection during early boot time.  After all, there are no
concurrent tasks, so there can be no grace periods completing
concurrently.

But this does need an Acked-by from the cgroups folks.
Located-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: mathieu.desnoyers@polymtl.ca
Cc: josh@joshtriplett.org
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
Cc: rostedt@goodmis.org
Cc: Valdis.Kletnieks@vt.edu
Cc: dhowells@redhat.com
LKML-Reference: <1267311188-16603-2-git-send-email-paulmck@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/linux/cgroup.h

@@ -489,6 +489,7 @@ static inline struct cgroup_subsys_state *task_subsys_state(
 {
 	return rcu_dereference_check(task->cgroups->subsys[subsys_id],
 				     rcu_read_lock_held() ||
+				     !rcu_scheduler_active ||
 				     cgroup_lock_is_held());
 }