rhashtable-test: Use walker to test bucket statistics

As resizes may continue to run in the background, use walker to
ensure we see all entries. Also print the encountered number
of rehashes queued up while traversing.

This may lead to warnings due to entries being seen multiple
times. We consider them non-fatal.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>