rcu: Update RCU maintainership

Drop Dipankar Sarma at his request (https://lkml.org/lkml/2014/6/2/628),
add Josh Triplett based on long-term review, contributions, and
agreement to take on this role (https://lkml.org/lkml/2014/6/2/554

).

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