rcu: Four-level hierarchy is no longer experimental

Time to make the four-level-hierarchy setting less scary, so this
commit removes "Experimental" from the boot-time message.  Leave the
message in order to get a heads-up on any possible need to expand to
a five-level hierarchy.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcutree_plugin.h

@@ -68,7 +68,7 @@ static void __init rcu_bootup_announce_oddness(void)
 	printk(KERN_INFO "\tAdditional per-CPU info printed with stalls.\n");
 #endif
 #if NUM_RCU_LVL_4 != 0
-	printk(KERN_INFO "\tExperimental four-level hierarchy is enabled.\n");
+	printk(KERN_INFO "\tFour-level hierarchy is enabled.\n");
 #endif
 	if (rcu_fanout_leaf != CONFIG_RCU_FANOUT_LEAF)
 		printk(KERN_INFO "\tExperimental boot-time adjustment of leaf fanout to %d.\n", rcu_fanout_leaf);