rcu-tasks: Handle the running-offline idle-task special case

The idle task corresponding to an offline CPU can appear to be running
while that CPU is offline.  This commit therefore adds checks for this
situation, treating it as a quiescent state.  Because the tasklist scan
and the holdout-list scan now exclude CPU-hotplug operations, readers
on the CPU-hotplug paths are still waited for.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>