rcu: Add designated reviewers for RCU

Adding Steven Rostedt, Mathieu Desnoyers, and Lai Jiangshan as designated
RCU reviewers based on recent emails:

o	https://lkml.org/lkml/2014/6/2/578 (Steven)
o	https://lkml.org/lkml/2014/6/2/621 (Mathieu)
o	https://lkml.org/lkml/2014/6/3/897

 (Lai)

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>