selftests/bpf: Test that module can't be unloaded with attached trampoline

Adding test to verify that once we attach module's trampoline,
the module can't be unloaded.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210414195147.1624932-7-jolsa@kernel.org