selftests/bpf: Make test_bench_attach serial

selftests/bpf: Make test_bench_attach serial

Alexei hit another rcu warnings because of this test.

Making test_bench_attach serial so it does not disrupts
other tests during parallel tests run.

While this change is not the fix, it should be less likely
to hit it with this test being executed serially.

Reported-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20221116100228.2064612-2-jolsa@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>