lwtunnel: add options setting and dumping for vxlan

Based on the code framework built on the last patch, to
support setting and dumping for vxlan, we only need to
add ip_tun_parse_opts_vxlan() for .build_state and
ip_tun_fill_encap_opts_vxlan() for .fill_encap and
if (tun_flags & TUNNEL_VXLAN_OPT) for .get_encap_size.

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