rcu: Consolidate duplicate callback-list initialization

There are a couple of open-coded initializations of the rcu_data
structure's RCU callback list.  This commit therefore consolidates
them into a new init_callback_list() function.

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