smp: Optimize send_call_function_single_ipi()

Just like the ttwu_queue_remote() IPI, make use of _TIF_POLLING_NRFLAG
to avoid sending IPIs to idle CPUs.

[ mingo: Fix UP build bug. ]

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200526161907.953304789@infradead.org

kernel/sched/idle.c

@@ -289,6 +289,11 @@ static void do_idle(void)
 	 */
 	smp_mb__after_atomic();
 
+	/*
+	 * RCU relies on this call to be done outside of an RCU read-side
+	 * critical section.
+	 */
+	flush_smp_call_function_from_idle();
 	sched_ttwu_pending();
 	schedule_idle();