selftests/net: test extended BPF fanout mode

Test PACKET_FANOUT_EBPF by inserting a program into the the kernel
with bpf(), then attaching it to the fanout group. Observe the same
payload-based distribution as in the PACKET_FANOUT_CBPF test.

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