rcu: Fix TINY_RCU rcu_is_cpu_rrupt_from_idle check

The rcu_is_cpu_rrupt_from_idle() needs to allow for one interrupt level
from the idle loop, but TINY_RCU checks for a call from the idle loop
itself.  This commit fixes this issue.

Reported-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>