VXLAN: Fix vxlan stats handling.

Fixes bug in VXLAN code where is iptunnel_xmit() called with NULL
dev->tstats.
This bug was introduced in commit 6aed0c8b (tunnel: use
iptunnel_xmit() again).

Following patch fixes bug by setting dev->tstats. It uses ip_tunnel
module code to share stats function.

CC: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>