selftests/net: cleanup unused parameter in psock_fanout

sock_fanout_open no longer sets the size of packet_socket ring, so stop
passing the parameter.

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