rcu: Make large and small sysidle systems use same state machine

Currently, small systems move back into RCU_SYSIDLE_NOT from
RCU_SYSIDLE_SHORT and large systems do not.  This works because moving
aggressively to RCU_SYSIDLE_NOT affects only performance, not correctness,
and on small systems, the performance impact should be negligible.  That
said, this difference does make RCU a bit more complex, and RCU does not
seem to be suffering from any lack of complexity.  This commit therefore
adjusts small-system operation to match that of large systems, so that
the state never moves back to RCU_SYSIDLE_NOT from RCU_SYSIDLE_SHORT.

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

kernel/rcu/tree_plugin.h

@@ -2659,7 +2659,8 @@ static void rcu_sysidle(unsigned long j)
 static void rcu_sysidle_cancel(void)
 {
 	smp_mb();
-	ACCESS_ONCE(full_sysidle_state) = RCU_SYSIDLE_NOT;
+	if (full_sysidle_state > RCU_SYSIDLE_SHORT)
+		ACCESS_ONCE(full_sysidle_state) = RCU_SYSIDLE_NOT;
 }
 
 /*