rcu: Do any deferred nocb wakeups at CPU offline time

Because the need to wake a nocb GP kthread ("rcuog") is sometimes
detected when wakeups cannot be done, these wakeups can be deferred.
The wakeups are then carried out by calls to do_nocb_deferred_wakeup()
at various safe points in the code, including RCU's idle hooks.  However,
when a CPU goes offline, it invokes arch_cpu_idle_dead() without invoking
any of RCU's idle hooks.

This commit therefore adds a call to do_nocb_deferred_wakeup() in
rcu_report_dead() in order to handle any deferred wakeups that have been
requested by the outgoing CPU.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tree.c

@@ -4178,6 +4178,9 @@ void rcu_report_dead(unsigned int cpu)
 	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
 	struct rcu_node *rnp = rdp->mynode;  /* Outgoing CPU's rdp & rnp. */
 
+	// Do any dangling deferred wakeups.
+	do_nocb_deferred_wakeup(rdp);
+
 	/* QS for any half-done expedited grace period. */
 	preempt_disable();
 	rcu_report_exp_rdp(this_cpu_ptr(&rcu_data));