rcu: Consistent rcu_is_watching() naming

The old rcu_is_cpu_idle() function is just __rcu_is_watching() with
preemption disabled.  This commit therefore renames rcu_is_cpu_idle()
to rcu_is_watching.

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