virtio_net: skb_orphan() and nf_reset() in xmit path.

The complex transmit free logic was introduced to avoid hangs on
removing the ip_conntrack module and also because drivers aren't
generally supposed to keep stale skbs for unbounded times.

After some debate, it was decided that while doing skb_orphan()
generally is a rat's nest, we can do it in this driver.  Following
patches take advantage of this.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>