bpf: selftests add sockmap tests

This generates a set of sockets, attaches BPF programs, and sends some
simple traffic using basic send/recv pattern. Additionally, we do a bunch
of negative tests to ensure adding/removing socks out of the sockmap fail
correctly.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>