test_rhashtable: Remove bogus max_size setting

Now that resizing is completely automatic, we need to remove
the max_size setting or the test will fail.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>