rcu: Remove rcu_needs_cpu_flush() to avoid false quiescent states

The purpose of rcu_needs_cpu_flush() was to iterate on pushing the
current grace period in order to help the current CPU enter dyntick-idle
mode.  However, this can result in failures if the CPU starts entering
dyntick-idle mode, but then backs out.  In this case, the call to
rcu_pending() from rcu_needs_cpu_flush() might end up announcing a
non-existing quiescent state.

This commit therefore removes rcu_needs_cpu_flush() in favor of letting
the dyntick-idle machinery at the end of the softirq handler push the
loop along via its call to rcu_pending().

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

kernel/rcutree.c

@@ -1528,9 +1528,6 @@ static void rcu_process_callbacks(struct softirq_action *unused)
 				&__get_cpu_var(rcu_sched_data));
 	__rcu_process_callbacks(&rcu_bh_state, &__get_cpu_var(rcu_bh_data));
 	rcu_preempt_process_callbacks();
-
-	/* If we are last CPU on way to dyntick-idle mode, accelerate it. */
-	rcu_needs_cpu_flush();
 	trace_rcu_utilization("End RCU core");
 }