tc: m_tunnel_key: add options support for vxlan

tc: m_tunnel_key: add options support for vxlan

This patch is to add TCA_TUNNEL_KEY_ENC_OPTS_VXLAN's parse and
print to implement vxlan options support in m_tunnel_key, like
Commit 6217917a3826 ("tc: m_tunnel_key: Add tunnel option support
to act_tunnel_key") for geneve options support.

Option is expressed a 32bit number for gbp only, and vxlan
doesn't support multiple options.

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

  # ip link add name vxlan1 type vxlan dstport 0 external
  # tc qdisc add dev eth0 ingress
  # tc filter add dev eth0 protocol ip parent ffff: \
      flower indev eth0 \
        ip_proto udp \
        action tunnel_key \
          set src_ip 10.0.99.192 \
          dst_ip 10.0.99.193 \
          dst_port 6081 \
          id 11 \
          vxlan_opts 65793 \
      action mirred egress redirect dev vxlan1
  # tc -s filter show dev eth0 parent ffff:

     filter protocol ip pref 49152 flower chain 0 handle 0x1
       indev eth0
       eth_type ipv4
       ip_proto udp
       not_in_hw
         action order 1: tunnel_key  set
         src_ip 10.0.99.192
         dst_ip 10.0.99.193
         key_id 11
         dst_port 6081
         vxlan_opts 65793
         ...

v1->v2:
  - get_u32 with base = 0 for gbp.
  - use to print_unint("0x%x") to print gbp.
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>