virtio_net: Reuse buffer free function

virtnet_rq_free_unused_buf() helper function to free the buffer
already exists. Avoid code duplication by reusing existing function.

Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Signed-off-by: Parav Pandit <parav@nvidia.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>