virtio_net: Call dev_kfree_skb_any instead of dev_kfree_skb.

Replace dev_kfree_skb with dev_kfree_skb_any in start_xmit which can
be called in hard irq and other contexts.

start_xmit only frees skbs that it is dropping.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>