Add support to dump netkit link information to bpftool in similar way as
we have for XDP. The netkit link info only exposes the ifindex and the
attach_type.

Below shows an example link dump output, and a cgroup link is included for
comparison, too:

  # bpftool link
  [...]
  10: cgroup  prog 2466
        cgroup_id 1  attach_type cgroup_inet6_post_bind
  [...]
  8: netkit  prog 35
        ifindex nk1(18)  attach_type netkit_primary
  [...]

Equivalent json output:

  # bpftool link --json
  [...]
  {
    "id": 10,
    "type": "cgroup",
    "prog_id": 2466,
    "cgroup_id": 1,
    "attach_type": "cgroup_inet6_post_bind"
  },
  [...]
  {
    "id": 12,
    "type": "netkit",
    "prog_id": 61,
    "devname": "nk1",
    "ifindex": 21,
    "attach_type": "netkit_primary"
  }
  [...]
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20231024214904.29825-5-daniel@iogearbox.netSigned-off-by: Martin KaFai Lau <martin.lau@kernel.org>