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It is possible for bpf_kfunc_call_test_release() to be called from
bpf_map_free_deferred() when bpf_testmod is already unloaded and
perf_test_stuct.cnt which it tries to decrease is no longer in memory.
This patch tries to fix the issue by waiting for all references to be
dropped in bpf_testmod_exit().

The issue can be triggered by running 'test_progs -t map_kptr' in 6.5,
but is obscured in 6.6 by d119357d ("rcu-tasks: Treat only
synchronous grace periods urgently").

Fixes: 65eb006d

 ("bpf: Move kernel test kfuncs to bpf_testmod")
Signed-off-by: Artem Savkov <asavkov@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Cc: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/82f55c0e-0ec8-4fe1-8d8c-b1de07558ad9@linux.dev
Link: https://lore.kernel.org/bpf/20240110085737.8895-1-asavkov@redhat.com

Signed-off-by: Alexei Starovoitov <ast@kernel.org>