rcu: Fix CPU offload boot message when no CPUs are offloaded

In CONFIG_RCU_NOCB_CPU=y kernels, if the boot parameters indicate that
none of the CPUs should in fact be offloaded, the following somewhat
obtuse message appears:

	Offload RCU callbacks from CPUs: .

This commit therefore makes the message at least grammatically correct
in this case:

	Offload RCU callbacks from CPUs: (none)
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree_plugin.h

@@ -2312,6 +2312,9 @@ void __init rcu_init_nohz(void)
 		cpumask_and(rcu_nocb_mask, cpu_possible_mask,
 			    rcu_nocb_mask);
 	}
+	if (cpumask_empty(rcu_nocb_mask))
+		pr_info("\tOffload RCU callbacks from CPUs: (none).\n");
+	else
 		pr_info("\tOffload RCU callbacks from CPUs: %*pbl.\n",
 			cpumask_pr_args(rcu_nocb_mask));
 	if (rcu_nocb_poll)