bpftool: Implement link show support for netkit

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.net
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>