s390/smp: only send external call ipi if needed

If the per cpu ec_mask bit of the receiving cpu is already set there is
no need to send an ipi, since a different cpu has already sent an ipi
and the receiving cpu has not yet executed the external call ipi handler.
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/smp.c

@@ -159,9 +159,9 @@ static void pcpu_ec_call(struct pcpu *pcpu, int ec_bit)
 {
 	int order;
 
-	set_bit(ec_bit, &pcpu->ec_mask);
-	order = pcpu_running(pcpu) ?
-		SIGP_EXTERNAL_CALL : SIGP_EMERGENCY_SIGNAL;
+	if (test_and_set_bit(ec_bit, &pcpu->ec_mask))
+		return;
+	order = pcpu_running(pcpu) ? SIGP_EXTERNAL_CALL : SIGP_EMERGENCY_SIGNAL;
 	pcpu_sigp_retry(pcpu, order, 0);
 }