hmap: Make bad hash functions easier to find.

hmap: Make bad hash functions easier to find.

The hmap code has for a long time incremented a counter when a hash bucket
grew to have many entries.  This can let a developer know that some hash
function is performing poorly, but doesn't give any hint as to which one.
This commit improves the situation by adding rate-limited debug logging
that points out a particular line of code as the source of the poor hash
behavior.  It should make issues easier to track down.

Bug #19926.
Signed-off-by: Ben Pfaff <blp@nicira.com>
Lauded-by: Keith Amidon <keith@nicira.com>
Acked-by: Justin Pettit <jpettit@nicira.com>