sched: Add missing rcu_dereference() around ->real_parent usage

Wrap another ->real_parent dereference while under rcu_read_lock.

Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Glauber Costa <glommer@parallels.com>
Cc: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Link: http://lkml.kernel.org/r/20111215164918.GA13003@www.outflux.net
[ tidied up the changelog ]
Signed-off-by: Ingo Molnar <mingo@elte.hu>