Improve random distribution for an hmap with a small number of nodes
with the expense of the increased cpu cost.
It would be a fair trade-off because the situation is rather common
for bond, which is currently the only consumer of this API in tree.
Consider 2 items, 4 buckets, no collision.
bucket 0 item 0
bucket 1
bucket 2
bucket 3 item 1
The old algorithm picks item 0 if rand % 4 == 0. (25%)
Otherwise it picks item 1. (75%)
This change makes them 50%.
Acked-by: Ben Pfaff <blp@nicira.com> Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>