selftests/net: expect headroom in psock_fanout rollover

psock_fanout tests the various fanout modes. Change the test for
rollover mode to expect early rollover due to socket pressure
as implemented in 2ccdbaa6 ("packet: rollover lock contention
avoidance").

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