selftests/net: fixes psock_fanout eBPF test case

eBPF test fails due to verifier failure because log_buf is too small.
Fixed by increasing log_buf size

Signed-off-by: Prashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>