xen-netfront: be more drop monitor friendly

xennet_start_xmit() might copy skb with inappropriate layout
into a fresh one.

Old skb is freed, and at this point it is not a drop, but
a consume. New skb will then be either consumed or dropped.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>