lwtunnel: add options setting and dumping for geneve

To add options setting and dumping, .build_state(), .fill_encap() and
.get_encap_size() in ip_tun_lwt_ops needs to be extended:

ip_tun_build_state():
  ip_tun_parse_opts():
    ip_tun_parse_opts_geneve()

ip_tun_fill_encap_info():
  ip_tun_fill_encap_opts():
    ip_tun_fill_encap_opts_geneve()

ip_tun_encap_nlsize()
   ip_tun_opts_nlsize():
     if (tun_flags & TUNNEL_GENEVE_OPT)

ip_tun_parse_opts(), ip_tun_fill_encap_opts() and ip_tun_opts_nlsize()
processes LWTUNNEL_IP_OPTS.

ip_tun_parse_opts_geneve(), ip_tun_fill_encap_opts_geneve() and
if (tun_flags & TUNNEL_GENEVE_OPT) processes LWTUNNEL_IP_OPTS_GENEVE.

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