linuxmodule/todevce: don't attempt to defer packet after dev_queue_xmit

After dev_queue_xmit, the packet has been consumed regardless of ret. If ret
were not zero, _q = p despite this fact and it will be Packet::kill'd by
ToDevice::cleanup.  We could skb_clone but that seems wasteful; instead,
drop.

We could also skb_get and then consume_skb on success except that there are
some network drivers (e.g. ppp_generic) that assume the skb it unshared and
will BUG otherwise.