tunnel: use iptunnel_xmit() again

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 <[email protected]>
Cc: Stephen Hemminger <[email protected]>
Cc: "David S. Miller" <[email protected]>
Signed-off-by: Cong Wang <[email protected]>
Acked-by: Eric Dumazet <[email protected]>
Signed-off-by: David S. Miller <[email protected]>