selftests/bpf: switch test_tcpnotify to perf_buffer API

Switch test_tcpnotify test to use libbpf's perf_buffer API instead of
re-implementing portion of it.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>