sched: provide scheduler_ipi() callback in response to smp_send_reschedule()

I visited existing smp_send_reschedule() implementations and tried to
add a call to scheduler_ipi() in their handler part, but esp. for MIPS
I'm not quite sure I actually got all of them.
 

diff --git a/arch/s390/kernel/smp.c b/arch/s390/kernel/smp.c
index 94cf510..61789e8 100644
--- a/arch/s390/kernel/smp.c
+++ b/arch/s390/kernel/smp.c

@@ -163,12 +163,12 @@ static void do_ext_call_interrupt(unsigned int ext_int_code,
 
 	/*
 	 * handle bit signal external calls
-	 *
-	 * For the ec_schedule signal we have to do nothing. All the work
-	 * is done automatically when we return from the interrupt.
 	 */
 	bits = xchg(&S390_lowcore.ext_call_fast, 0);
 
+	if (test_bit(ec_schedule, &bits))
+		scheduler_ipi();
+
 	if (test_bit(ec_call_function, &bits))
 		generic_smp_call_function_interrupt();