net: l2tp: only set L2TP_ATTR_UDP_CSUM if AF_INET

Only set L2TP_ATTR_UDP_CSUM in l2tp_nl_tunnel_send()
when it's running over IPv4.

This prepares the code to also have IPv6 specific attributes.

Signed-off-by: Asbjoern Sloth Toennesen <asbjorn@asbjorn.st>
Signed-off-by: David S. Miller <davem@davemloft.net>