{f,m}_bpf: add more example code

{f,m}_bpf: add more example code

I've added three examples to examples/bpf/ that demonstrate how one can
implement eBPF tail calls in tc with f.e. multiple levels of nesting.
That should act as a good starting point, but also as test cases for the
ELF loader and kernel. A real test suite for {f,m,e}_bpf is still to be
developed in future work.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>