vxlan: Update tx_errors statistics if vxlan_build_skb return err.

If vxlan_build_skb return err < 0, tx_errors should be also increased.

Signed-off-by: Haishuang Yan <yanhaishuang@cmss.chinamobile.com>
Acked-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>