selftests/net: test classic bpf fanout mode

Test PACKET_FANOUT_CBPF by inserting a cBPF program that selects a
socket by payload. Requires modifying the test program to send
packets with multiple payloads.

Also fix a bug in testing the return value of mmap()

Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>