rcu: Exclude RCU-offline CPUs from expedited grace periods

The expedited RCU grace periods currently rely on a failure indication
from smp_call_function_single() to determine that a given CPU is offline.
This works after a fashion, but is more contorted and less precise than
relying on RCU's internal state.  This commit therefore takes a first
step towards relying on internal state.

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