iproute_lwtunnel: add options support for erspan metadata

iproute_lwtunnel: add options support for erspan metadata

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

Option is expressed as version:index:dir:hwid, dir and hwid will be parsed
when version is 2, while index will be parsed when version is 1. All of
these are numbers. erspan doesn't support multiple options.

With this patch, users can add and dump erspan 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 erspan1 type erspan key 1 seq erspan 123 \
    local 10.1.0.2 remote 10.1.0.1
  # ip -n b addr add 1.1.1.1/24 dev erspan1
  # ip -n b link set erspan1 up
  # ip -n b route add 2.1.1.0/24 dev erspan1
  # ip -n a link add erspan1 type erspan key 1 seq local 10.1.0.1 external
  # ip -n a addr add 2.1.1.1/24 dev erspan1
  # ip -n a link set erspan1 up
  # ip -n a route add 1.1.1.0/24 encap ip id 1 \
    erspan_opts 2:123:1:2 dst 10.1.0.2 dev erspan1
  # 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
     erspan_opts 2:0:1:2 dev erspan1 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=0.124 ms

v1->v2:
  - improve the changelog.
  - use PRINT_ANY to support dumping with json format.
v2->v3:
  - implement proper JSON object for opts instead of just bunch of strings.
v3->v4:
  - keep the same format between input and output, json and non json.
  - print version, index, dir and hwid as uint.

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