xen: Validate online cpus in set_affinity

The user space interface does not filter out offline cpus. It merily
verifies that the mask contains at least one online cpu. So the
selector in the irq chip implementation needs to make sure to pick
only an online cpu because otherwise:

     Offline Core 1
     Set affinity to 0xe
     Selector will pick first set bit, i.e. core 1

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Xen <xen-devel@lists.xenproject.org>
Link: http://lkml.kernel.org/r/20140304203100.978031089@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

drivers/xen/events/events_base.c

@@ -1324,7 +1324,7 @@ static int rebind_irq_to_cpu(unsigned irq, unsigned tcpu)
 static int set_affinity_irq(struct irq_data *data, const struct cpumask *dest,
 			    bool force)
 {
-	unsigned tcpu = cpumask_first(dest);
+	unsigned tcpu = cpumask_first_and(dest, cpu_online_mask);
 
 	return rebind_irq_to_cpu(data->irq, tcpu);
 }