hw/virtio: Acquire RCU read lock in virtqueue_packed_drop_all()

hw/virtio: Acquire RCU read lock in virtqueue_packed_drop_all()

vring_get_region_caches() must be called with the RCU read lock
acquired. virtqueue_packed_drop_all() does not, and uses the
'caches' pointer. Fix that by using the RCU_READ_LOCK_GUARD()
macro.

Reported-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210906104318.1569967-3-philmd@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>