rcu: Eliminate callback-invocation/invocation use of rsp

Now that there is only one rcu_state structure, there is less point in
maintaining a pointer to it.  This commit therefore replaces rsp with
&rcu_state in rcu_do_batch(), invoke_rcu_callbacks(), and __call_rcu().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>