vsock/virtio: fix DMA alignment for event_list

vsock/virtio: fix DMA alignment for event_list

On non-cache-coherent platforms, when a structure contains a buffer
used for DMA alongside fields that the CPU writes to, cacheline sharing
can cause data corruption.

The event_list array is used for DMA_FROM_DEVICE operations via
virtqueue_add_inbuf(). The adjacent event_run and guest_cid fields are
written by the CPU while the buffer is available, so mapped for the
device. If these share cachelines with event_list, CPU writes can
corrupt DMA data.

Add __dma_from_device_group_begin()/end() annotations to ensure event_list
is isolated in its own cachelines.

Message-ID: <f19ebd74f70c91cab4b0178df78cf6a6e107a96b.1767601130.git.mst@redhat.com>
Acked-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>