sched: Fix sched_domain iterations vs. RCU

Vladis Kletnieks reported a new RCU debug warning in the scheduler.

Since commit dce840a0 ("sched: Dynamically allocate sched_domain/
sched_group data-structures") the sched_domain trees are protected by
RCU instead of RCU-sched.

This means that we need to include rcu_read_lock() protection when we
iterate them since disabling preemption doesn't suffice anymore.

Reported-by:  <Valdis.Kletnieks@vt.edu>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1302882741.2388.241.camel@twins


Signed-off-by: Ingo Molnar <mingo@elte.hu>