selftests/bpf: Fix potential premature unload in bpf_testmod

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 d119357d07435 ("rcu-tasks: Treat only
synchronous grace periods urgently").

Fixes: 65eb006d85a2 ("bpf: Move kernel test kfuncs to bpf_testmod")
Signed-off-by: Artem Savkov <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Acked-by: Yonghong Song <[email protected]>
Cc: Jiri Olsa <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]
Link: https://lore.kernel.org/bpf/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

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