hmap: Improve debug log message when reporting unusually large buckets.

hmap: Improve debug log message when reporting unusually large buckets.

I was seeing a lot of these messages, including a lot of them suppressed
by rate-limiting, and I wondered whether any really big messages were
being suppressed.  By reporting the largest bucket, instead of just every
large bucket, it becomes more likely that the truly too-large buckets get
reported.

(The problem I saw was a false alarm.)

Acked-by: Han Zhou <hzhou8@ebay.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>