bpf: Emit better log message if bpf_iter ctx arg btf_id == 0

To avoid kernel build failure due to some missing .BTF-ids referenced
functions/types, the patch ([1]) tries to fill btf_id 0 for
these types.

In bpf verifier, for percpu variable and helper returning btf_id cases,
verifier already emitted proper warning with something like
  verbose(env, "Helper has invalid btf_id in R%d\n", regno);
  verbose(env, "invalid return type %d of func %s#%d\n",
          fn->ret_type, func_id_name(func_id), func_id);

But this is not the case for bpf_iter context arguments.
I hacked resolve_btfids to encode btf_id 0 for struct task_struct.
With `./test_progs -n 7/5`, I got,
  0: (79) r2 = *(u64 *)(r1 +0)
  func 'bpf_iter_task' arg0 has btf_id 29739 type STRUCT 'bpf_iter_meta'
  ; struct seq_file *seq = ctx->meta->seq;
  1: (79) r6 = *(u64 *)(r2 +0)
  ; struct task_struct *task = ctx->task;
  2: (79) r7 = *(u64 *)(r1 +8)
  ; if (task == (void *)0) {
  3: (55) if r7 != 0x0 goto pc+11
  ...
  ; BPF_SEQ_PRINTF(seq, "%8d...