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>