iproute_lwtunnel: add options support for vxlan metadata

iproute_lwtunnel: add options support for vxlan metadata

This patch is to add LWTUNNEL_IP_OPTS_VXLAN's parse and print to implement
vxlan options support in iproute_lwtunnel.

Option is expressed a number for gbp only, and vxlan doesn't support
multiple options.

With this patch, users can add and dump vxlan options like:

  # ip netns add a
  # ip netns add b
  # ip -n a link add eth0 type veth peer name eth0 netns b
  # ip -n a link set eth0 up
  # ip -n b link set eth0 up
  # ip -n a addr add 10.1.0.1/24 dev eth0
  # ip -n b addr add 10.1.0.2/24 dev eth0
  # ip -n b link add vxlan1 type vxlan id 1 local 10.1.0.2 \
    remote 10.1.0.1 dev eth0 ttl 64 gbp
  # ip -n b addr add 1.1.1.1/24 dev vxlan1
  # ip -n b link set vxlan1 up
  # ip -n b route add 2.1.1.0/24 dev vxlan1
  # ip -n a link add vxlan1 type vxlan local 10.1.0.1 dev eth0 ttl 64 \
    gbp external
  # ip -n a addr add 2.1.1.1/24 dev vxlan1
  # ip -n a link set vxlan1 up
  # ip -n a route add 1.1.1.0/24 encap ip id 1 \
    vxlan_opts 1110 dst 10.1.0.2 dev vxlan1
  # ip -n a route show
  # ip netns exec a ping 1.1.1.1 -c 1

   1.1.1.0/24  encap ip id 1 src 0.0.0.0 dst 10.1.0.2 ttl 0 tos 0
     vxlan_opts 1110 dev vxlan1 scope link

   PING 1.1.1.1 (1.1.1.1) 56(84) bytes of data.
   64 bytes from 1.1.1.1: icmp_seq=1 ttl=64 time=0.111 ms

v1->v2:
  - improve the changelog.
  - get_u32 with base = 0 for gbp.
  - use PRINT_ANY to support dumping with json format.
v2->v3:
  - implement proper JSON array for opts.
v3->v4:
  - keep the same format between input and output, json and non json.
  - print gbp as uint.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David Ahern <dsahern@gmail.com>