bpf, tests: add a test for htab lookup + update traversal

Add a test case to track behaviour when traversing and updating the
htab map. We recently used such traversal, so it's quite useful to
keep it as an example in selftests.

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