bond: raise minimal bond distribution per interface

Raise the minimal per interface packet distribution from 7 to 24.

With 256 packet distributing to 3 interfaces, the expected packets per
interface should be 256/3 = 85.3

Tested with 200 runs, the average number of packet sent to a single
interface is 85.9. close to the expected number, standard deviation
within the 200 run is 24.4. Tested with 2x standard deviation with
10K test runs, got around 0.1% failure rate. 2.5x standard deviation
passes 100K test runs without failure.

Using 2.5x for the unit test, 83.5 - 2.5 * 24.4, Round down to the
whole number of 24.

Signed-off-by: Andy Zhou <azhou@nicira.com>
Reviewed-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>