bpftool: Add support to display uprobe_multi links

Adding support to display details for uprobe_multi links,
both plain:

  # bpftool link -p
  ...
  24: uprobe_multi  prog 126
          uprobe.multi  path /home/jolsa/bpf/test_progs  func_cnt 3  pid 4143
          offset             ref_ctr_offset     cookies
          0xd1f88            0xf5d5a8           0xdead
          0xd1f8f            0xf5d5aa           0xbeef
          0xd1f96            0xf5d5ac           0xcafe

and json:

  # bpftool link -p
  [{
  ...
      },{
          "id": 24,
          "type": "uprobe_multi",
          "prog_id": 126,
          "retprobe": false,
          "path": "/home/jolsa/bpf/test_progs",
          "func_cnt": 3,
          "pid": 4143,
          "funcs": [{
                  "offset": 860040,
                  "ref_ctr_offset": 16111016,
                  "cookie": 57005
              },{
                  "offset": 860047,
                  "ref_ctr_offset": 16111018,
                  "cookie": 48879
              },{
                  "offset": 860054,
                  "ref_ctr_offset": 16111020,
                  "cookie": 51966
              }
          ]
      }
  ]

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Acked-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/bpf/20231125193130.834322-7-jolsa@kernel.org