time: RCU permitted to stop idle entry via softirq

RCU needs to be able to use softirq to stop idle entry in order to
be able to drain RCU callbacks from the current CPU, which in turn
enables faster entry into dyntick-idle mode, which in turn reduces power
consumption.  This commit therefore silences the error message that is
sometimes produced when the going-idle CPU suddenly finds that it has
an RCU_SOFTIRQ to process.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
backported by faux123 from Linux 3.5+ to Linux 3.0