rhashtable: allow to unload test module

There's no good reason why to disallow unloading of the rhashtable
test case module.

Commit 9d6dbe1b moved the code from a boot test into a stand-alone
module, but only converted the subsys_initcall() handler into a
module_init() function without a related exit handler, and thus
preventing the test module from unloading.

Fixes: 9d6dbe1b ("rhashtable: Make selftest modular")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>