Add a local kptr test with no special fields in the struct. Without the
previous patch, the following warning will hit:

  [   44.683877] WARNING: CPU: 3 PID: 485 at kernel/bpf/syscall.c:660 bpf_obj_free_fields+0x220/0x240
  [   44.684640] Modules linked in: bpf_testmod(OE)
  [   44.685044] CPU: 3 PID: 485 Comm: kworker/u8:5 Tainted: G           OE      6.5.0-rc5-01703-g260d855e9b90 #248
  [   44.685827] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
  [   44.686693] Workqueue: events_unbound bpf_map_free_deferred
  [   44.687297] RIP: 0010:bpf_obj_free_fields+0x220/0x240
  [   44.687775] Code: e8 55 17 1f 00 49 8b 74 24 08 4c 89 ef e8 e8 14 05 00 e8 a3 da e2 ff e9 55 fe ff ff 0f 0b e9 4e fe ff
                       ff 0f 0b e9 47 fe ff ff <0f> 0b e8 d9 d9 e2 ff 31 f6 eb d5 48 83 c4 10 5b 41 5c e
  [   44.689353] RSP: 0018:ffff888106467cb8 EFLAGS: 00010246
  [   44.689806] RAX: 0000000000000000 RBX: ffff888112b3a200 RCX: 0000000000000001
  [   44.690433] RDX: 0000000000000000 RSI: dffffc0000000000 RDI: ffff8881128ad988
  [   44.691094] RBP: 0000000000000002 R08: ffffffff81370bd0 R09: 1ffff110216231a5
  [   44.691643] R10: dffffc0000000000 R11: ffffed10216231a6 R12: ffff88810d68a488
  [   44.692245] R13: ffff88810767c288 R14: ffff88810d68a400 R15: ffff88810d68a418
  [   44.692829] FS:  0000000000000000(0000) GS:ffff8881f7580000(0000) knlGS:0000000000000000
  [   44.693484] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  [   44.693964] CR2: 000055c7f2afce28 CR3: 000000010fee4002 CR4: 0000000000370ee0
  [   44.694513] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
  [   44.695102] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
  [   44.695747] Call Trace:
  [   44.696001]  <TASK>
  [   44.696183]  ? __warn+0xfe/0x270
  [   44.696447]  ? bpf_obj_free_fields+0x220/0x240
  [   44.696817]  ? report_bug+0x220/0x2d0
  [   44.697180]  ? handle_bug+0x3d/0x70
  [   44.697507]  ? exc_invalid_op+0x1a/0x50
  [   44.697887]  ? asm_exc_invalid_op+0x1a/0x20
  [   44.698282]  ? btf_find_struct_meta+0xd0/0xd0
  [   44.698634]  ? bpf_obj_free_fields+0x220/0x240
  [   44.699027]  ? bpf_obj_free_fields+0x1e2/0x240
  [   44.699414]  array_map_free+0x1a3/0x260
  [   44.699763]  bpf_map_free_deferred+0x7b/0xe0
  [   44.700154]  process_one_work+0x46d/0x750
  [   44.700523]  worker_thread+0x49e/0x900
  [   44.700892]  ? pr_cont_work+0x270/0x270
  [   44.701224]  kthread+0x1ae/0x1d0
  [   44.701516]  ? kthread_blkcg+0x50/0x50
  [   44.701860]  ret_from_fork+0x34/0x50
  [   44.702178]  ? kthread_blkcg+0x50/0x50
  [   44.702508]  ret_from_fork_asm+0x11/0x20
  [   44.702880]  </TASK>

With the previous patch, there is no warnings.
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20230824063422.203097-1-yonghong.song@linux.devSigned-off-by: Alexei Starovoitov <ast@kernel.org>