selftests/bpf: Add tests for livepatch + bpf trampoline

Both livepatch and BPF trampoline use ftrace. Special attention is needed
when livepatch and fexit program touch the same function at the same
time, because livepatch updates a kernel function and the BPF trampoline
need to call into the right version of the kernel function.

Use samples/livepatch/livepatch-sample.ko for the test.

The test covers two cases:
  1) When a fentry program is loaded first. This exercises the
     modify_ftrace_direct code path.
  2) When a fentry program is loaded first. This exercises the
     register_ftrace_direct code path.

Signed-off-by: Song Liu <song@kernel.org>
Reviewed-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20251027175023.1521602-4-song@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>

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