virtio/s390: size of SET_IND payload

SET_IND takes as a payload the _address_ of the indicators, meaning
that we have one of the rare cases where kmalloc(sizeof(&...)) is
actually correct. Let's clarify that with a comment.

The count for the ccw, however, was only correct because the
indicators are 64 bit. Let's use the correct value.

Reported-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Reviewed-by: Halil Pasic <pasic@linux.vnet.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>