rcu: Avoid invoking RCU core on offline CPUs

Offline CPUs transition through the scheduler to the idle loop one
last time before being shut down.  This can result in RCU raising
softirq on this CPU, which is at best useless given that the CPU's
callbacks will be offloaded at CPU_DEAD time.  This commit therefore
avoids raising softirq on offline CPUs.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>