tunnel: use iptunnel_xmit() again

With recent patches from Pravin, most tunnels can't use iptunnel_xmit()
any more, due to ip_select_ident() and skb->ip_summed. But we can just
move these operations out of iptunnel_xmit(), so that tunnels can
use it again.

This by the way fixes a bug in vxlan (missing nf_reset()) for net-next.

Cc: Pravin B Shelar <pshelar@nicira.com>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv6/sit.c b/net/ipv6/sit.c
index 02f96dc..898e671 100644
--- a/net/ipv6/sit.c
+++ b/net/ipv6/sit.c
@@ -899,6 +899,8 @@
 	if ((iph->ttl = tiph->ttl) == 0)
 		iph->ttl	=	iph6->hop_limit;
 
+	skb->ip_summed = CHECKSUM_NONE;
+	ip_select_ident(iph, skb_dst(skb), NULL);
 	iptunnel_xmit(skb, dev);
 	return NETDEV_TX_OK;