hmap_random_node: Improve distribution

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>