virtio_ring: don't update event idx on get_buf

in virtio_net, if we disable the napi_tx, when we triger a tx interrupt,
the vq->event_triggered will be set to true. It will no longer be set to
false. Unless we explicitly call virtqueue_enable_cb_delayed or
virtqueue_enable_cb_prepare.

If we disable the napi_tx, it will only be called when the tx ring
buffer is relatively small.

Because event_triggered is true. Therefore, VRING_AVAIL_F_NO_INTERRUPT or
VRING_PACKED_EVENT_FLAG_DISABLE will not be set. So we update
vring_used_event(&vq->split.vring) or vq->packed.vring.driver->off_wrap
every time we call virtqueue_get_buf_ctx. This will bring more interruptions.

To summarize:
1) event_triggered was set to true in vring_interrupt()
2) after this nothing will happen for virtqueue_disable_cb() so
   VRING_AVAIL_F_NO_INTERRUPT is not set in avail_flags_shadow
3) virtqueue_get_buf_ctx_split() will still think the cb is enabled
   then it tries to publish new event

To fix, if event_triggered is set to true, do not update
vring_used_event(&vq->split.vring) or vq->packed.vring.driver->off_wrap

Tested with iperf:
iperf3 tcp stream:
vm1 -----------------> vm2
vm2 just receives tcp data stream from vm1, and sends the ack to vm1,
there are many tx interrupts in vm2.
but without event_triggered there are just a few tx interrupts.

Fixes: 8d622d21 ("virtio: fix up virtio_disable_cb")
Signed-off-by: Albert Huang <huangjie.albert@bytedance.com>
Message-Id: <20230321085953.24949-1-huangjie.albert@bytedance.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>