virtio_pci: Clear stale cpumask when setting irq affinity

commit 210d150e upstream.

The cpumask vp_dev->msix_affinity_masks[info->msix_vector] may contain
staled information when vp_set_vq_affinity() gets called, so clear it
before setting the new cpu bit mask.
Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
[ luis: backported to 3.16:
  - file rename: virtio_pci_common.c -> virtio_pci.c ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

drivers/virtio/virtio_pci.c

@@ -645,6 +645,7 @@ static int vp_set_vq_affinity(struct virtqueue *vq, int cpu)
 		if (cpu == -1)
 			irq_set_affinity_hint(irq, NULL);
 		else {
+			cpumask_clear(mask);
 			cpumask_set_cpu(cpu, mask);
 			irq_set_affinity_hint(irq, mask);
 		}