openvswitch: fix vport packet length check.

When sending a packet to a tunnel device, the dev's hard_header_len
could be larger than the skb->len in function packet_length().
In the case of ip6gretap/erspan, hard_header_len = LL_MAX_HEADER + t_hlen,
which is around 180, and an ARP packet sent to this tunnel has
skb->len = 42.  This causes the 'unsign int length' to become super
large because it is negative value, causing the later ovs_vport_send
to drop it due to over-mtu size.  The patch fixes it by setting it to 0.

Signed-off-by: William Tu <u9012063@gmail.com>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>