tipc: add net device to skb before UDP xmit

Prior to this patch enabling a IPv4 UDP bearer caused a null pointer
dereference in iptunnel_xmit_stats(), when it tried to dereference the
net device from the skb. To resolve this we now point the skb device
to the net device resolved from the routing table.

Fixes: 039f5062 (ip_tunnel: Move stats update to iptunnel_xmit())
Signed-off-by: Richard Alpe <richard.alpe@ericsson.com>
Acked-by: Jon Maloy <jon.maloy@ericsson.com>
Reviewed-by: Erik Hugne <erik.hugne@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>