bpftool: Implement link show support for tcx

Add support to dump tcx link information to bpftool. This adds a
common helper show_link_ifindex_{plain,json}() which can be reused
also for other link types. The plain text and json device output is
the same format as in bpftool net dump.

Below shows an example link dump output along with a cgroup link
for comparison:

  # bpftool link
  [...]
  10: cgroup  prog 1977
        cgroup_id 1  attach_type cgroup_inet6_post_bind
  [...]
  13: tcx  prog 2053
        ifindex enp5s0(3)  attach_type tcx_ingress
  14: tcx  prog 2080
        ifindex enp5s0(3)  attach_type tcx_egress
  [...]

Equivalent json output:

  # bpftool link --json
  [...]
  {
    "id": 10,
    "type": "cgroup",
    "prog_id": 1977,
    "cgroup_id": 1,
    "attach_type": "cgroup_inet6_post_bind"
  },
  [...]
  {
    "id": 13,
    "type": "tcx",
    "prog_id": 2053,
    "devname": "enp5s0",
    "ifindex": 3,
    "attach_type": "tcx_ingress"
  },
  {
    "id": 14,
    "type": "tcx",
    "prog_id": 2080,
    "devname": "enp5s0",
    "ifindex": 3,
    "attach_type": "tcx_egress"
  }
  [...]

Suggested-by: Yafang Shao <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Reviewed-by: Quentin Monnet <[email protected]>
Acked-by: Yafang Shao <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Martin KaFai Lau <[email protected]>

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