Add call_rcu() interface

Adds call_rcu(), with RCU threads to invoke the callbacks.  By default,
there will be one such RCU thread per process, created the first time
that call_rcu() is invoked.  On systems supporting sched_getcpu(), it
is possible to create one RCU thread per CPU by calling
create_all_cpu_call_rcu_data().

This version includes feedback from Mathieu Desnoyers.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>