rcu: Remove useless rcu_data_p when !PREEMPT_RCU

The related warning from gcc 6.0:

  In file included from kernel/rcu/tree.c:4630:0:
  kernel/rcu/tree_plugin.h:810:40: warning: ‘rcu_data_p’ defined but not used [-Wunused-const-variable]
   static struct rcu_data __percpu *const rcu_data_p = &rcu_sched_data;
                                          ^~~~~~~~~~

Also remove always redundant rcu_data_p in tree.c.
Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree.c

@@ -108,7 +108,6 @@ RCU_STATE_INITIALIZER(rcu_sched, 's', call_rcu_sched);
 RCU_STATE_INITIALIZER(rcu_bh, 'b', call_rcu_bh);
 
 static struct rcu_state *const rcu_state_p;
-static struct rcu_data __percpu *const rcu_data_p;
 LIST_HEAD(rcu_struct_flavors);
 
 /* Dump rcu_node combining tree at boot to verify correct setup. */

kernel/rcu/tree_plugin.h

@@ -807,7 +807,6 @@ void exit_rcu(void)
 #else /* #ifdef CONFIG_PREEMPT_RCU */
 
 static struct rcu_state *const rcu_state_p = &rcu_sched_state;
-static struct rcu_data __percpu *const rcu_data_p = &rcu_sched_data;
 
 /*
  * Tell them what RCU they are running.