virtio_net: ensure netdev_tx_reset_queue is called on tx ring resize

virtnet_tx_resize() flushes remaining tx skbs, requiring DQL counters to
be reset when flushing has actually occurred. Add
virtnet_sq_free_unused_buf_done() as a callback for virtqueue_reset() to
handle this.

Fixes: c8bd1f7f ("virtio_net: add support for Byte Queue Limits")
Cc: <stable@vger.kernel.org> # v6.11+
Signed-off-by: Koichiro Den <koichiro.den@canonical.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>