virtio_net: rename free_old_xmit_skbs to free_old_xmit

Since free_old_xmit_skbs not only deals with skb, but also xdp frame and
subsequent added xsk, so change the name of this function to
free_old_xmit.

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20240229072044.77388-19-xuanzhuo@linux.alibaba.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>