bpftool: Fix missing pids during link show

Current 'bpftool link' command does not show pids, e.g.,
  $ tools/build/bpftool/bpftool link
  ...
  4: tracing  prog 23
        prog_type lsm  attach_type lsm_mac
        target_obj_id 1  target_btf_id 31320

Hack the following change to enable normal libbpf debug output,
  --- a/tools/bpf/bpftool/pids.c
  +++ b/tools/bpf/bpftool/pids.c
  @@ -121,9 +121,9 @@ int build_obj_refs_table(struct hashmap **map, enum bpf_obj_type type)
          /* we don't want output polluted with libbpf errors if bpf_iter is not
           * supported
           */
  -       default_print = libbpf_set_print(libbpf_print_none);
  +       /* default_print = libbpf_set_print(libbpf_print_none); */
          err = pid_iter_bpf__load(skel);
  -       libbpf_set_print(default_print);
  +       /* libbpf_set_print(default_print); */

Rerun the above bpftool command:
  $ tools/build/bpftool/bpftool link
  libbpf: prog 'iter': BPF program load failed: Permission denied
  libbpf: prog 'iter': -- BEGIN PROG LOAD LOG --
  0: R1=ctx() R10=fp0
  ; struct task_struct *task = ctx->task; @ pid_iter.bpf.c:69
  0: (79) r6 = *(u64 *)(r1 +8)          ; R1=ctx() R6_w=ptr_or_null_task_struct(id=1)
  ; struct file *file = ctx->file; @ pid_iter.bpf.c:68
  ...
  ; struct bpf_link *link = (struct bpf_link *) file->private_data; @ pid_iter.bpf.c:103
  80: (79) r3 = *(u64 *)(r8 +432)       ; R3_w=scalar() R8=ptr_file()
  ; if (link->type == bpf_core_enum_value(enum bpf_link_type___local, @ pid_iter.bpf.c:105
  81: (61) r1 = *(u32 *)(r3 +12)
  R3 invalid mem access 'scalar'
  processed 39 insns (limit 1000000) max_states_per_insn 0 total_states 3 peak_states 3 mark_read 2
  -- END PROG LOAD LOG --
  libbpf: prog 'iter': failed to load: -13
  ...

The 'file->private_data' returns a 'void' type and this caused subsequent 'link->type'
(insn #81) failed in verification.

To fix the issue, do a type cast from 'void *' to 'struct bpf_link *' for file->private_data
as in this patch, and the 'bpftool link' runs successfully with 'pids'.
  $ tools/build/bpftool/bpftool link
  ...
  4: tracing  prog 23
        prog_type lsm  attach_type lsm_mac
        target_obj_id 1  target_btf_id 31320
        pids systemd(1)

Fixes: 44ba7b3 ("bpftool: Use a local copy of BPF_LINK_TYPE_PERF_EVENT in pid_iter.bpf.c")
Cc: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>

tools/bpf/bpftool/skeleton/pid_iter.bpf.c

@@ -100,7 +100,7 @@ int iter(struct bpf_iter__task_file *ctx)
 	if (obj_type == BPF_OBJ_LINK &&
 	    bpf_core_enum_value_exists(enum bpf_link_type___local,
 				       BPF_LINK_TYPE_PERF_EVENT___local)) {
-		struct bpf_link *link = (struct bpf_link *) file->private_data;
+		struct bpf_link *link = bpf_core_cast(file->private_data, struct bpf_link);
 
 		if (link->type == bpf_core_enum_value(enum bpf_link_type___local,
 						      BPF_LINK_TYPE_PERF_EVENT___local)) {