selftests/bpf: add l4 load balancer test based on sched_cls

this l4lb demo is a comprehensive test case for LLVM codegen and
kernel verifier. It's using fully inlined jhash(), complex packet
parsing and multiple map lookups of different types to stress
llvm and verifier.
The map sizes, map population and test vectors are artificial to
exercise different paths through the bpf program.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>