selftests/bpf: Fix map_kptr test failure

selftests/bpf: Fix map_kptr test failure

On my arm64 machine, I get the following failure:
  ...
  tester_init:PASS:tester_log_buf 0 nsec
  process_subtest:PASS:obj_open_mem 0 nsec
  process_subtest:PASS:specs_alloc 0 nsec
  serial_test_map_kptr:PASS:rcu_tasks_trace_gp__open_and_load 0 nsec
  ...
  test_map_kptr_success:PASS:map_kptr__open_and_load 0 nsec
  test_map_kptr_success:PASS:test_map_kptr_ref1 refcount 0 nsec
  test_map_kptr_success:FAIL:test_map_kptr_ref1 retval unexpected error: 2 (errno 2)
  test_map_kptr_success:PASS:test_map_kptr_ref2 refcount 0 nsec
  test_map_kptr_success:FAIL:test_map_kptr_ref2 retval unexpected error: 1 (errno 2)
  ...
  #201/21  map_kptr/success-map:FAIL

In serial_test_map_kptr(), before test_map_kptr_success(), one
kern_sync_rcu() is used to have some delay for freeing the map.
But in my environment, one kern_sync_rcu() seems not enough and
caused the test failure.

In bpf_map_free_in_work() in syscall.c, the queue time for
  queue_work(system_dfl_wq, &map->work)
may be longer than expected. This may cause the test failure
since test_map_kptr_success() expects all previous maps having been freed.

Since it is not clear how long queue_work() time takes, a bpf prog
is added to count the reference after bpf_kfunc_call_test_acquire().
If the number of references is 2 (for initial ref and the one just
acquired), all previous maps should have been released. This will
resolve the above 'retval unexpected error' issue.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/bpf/20260116052245.3692405-1-yonghong.song@linux.dev