geneve: Fix IPv6 xmit stats update.

Call to iptunnel_xmit_stats() is not required after udp-tunnel6-xmit.
By calling iptunnel_xmit_stats() results in incorrect device stats.
Following patch drops this call.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>