lib: test_rhashtable: Fix KASAN warning

I forgot one spot when introducing struct test_obj_val.

Fixes: e859afe1 ("lib: test_rhashtable: fix for large entry counts")
Reported by: kernel test robot <fengguang.wu@intel.com>
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>