ip_tunnel: allow not to count pkts on tstats by setting skb's dev to NULL

iptunnel_xmit() works as a common function, also used by a udp tunnel
which doesn't have to have a tunnel device, like how TIPC works with
udp media.

In these cases, we should allow not to count pkts on dev's tstats, so
that udp tunnel can work with no tunnel device safely.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>