correct mis-conversion set_bit() -> __cpumask_set_cpu() by 4aaca0e9cd

I have no idea how I came to use __cpumask_set_cpu() there, the
conversion should have been set_bit() -> __set_bit(). The wrong
construct results in problems on systems with relatively few CPUs.

Reported-by: Sander Eikelenboom <linux@xxxxxxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/common/softirq.c
+++ b/xen/common/softirq.c
@@ -106,7 +106,7 @@ void cpu_raise_softirq(unsigned int cpu,
     if ( !per_cpu(batching, this_cpu) || in_irq() )
         smp_send_event_check_cpu(cpu);
     else
-        __cpumask_set_cpu(nr, &per_cpu(batch_mask, this_cpu));
+        __set_bit(nr, &per_cpu(batch_mask, this_cpu));
 }
 
 void cpu_raise_softirq_batch_begin(void)