selftests/bpf: Add re-attach test to fentry_test

Adding the test to re-attach (detach/attach again) tracing
fentry programs, plus check that already linked program can't
be attached again.

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/20210414195147.1624932-4-jolsa@kernel.org