selftests/bpf: Fix trampoline_count clean up logic

Libbpf's Travis CI tests caught this issue. Ensure bpf_link and bpf_object
clean up is performed correctly.

Fixes: d633d579 ("selftest/bpf: Add test for allowed trampolines count")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20200220230546.769250-1-andriin@fb.com