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