blackhole_dev: convert self-test to KUnit

Convert this very simple smoke test to a KUnit test.

Add a missing `htons` call that was spotted[0] by kernel test robot
<lkp@intel.com> after initial conversion to KUnit.

Link: https://lore.kernel.org/oe-kbuild-all/202502090223.qCYMBjWT-lkp@intel.com/

 [0]
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Link: https://patch.msgid.link/20250208-blackholedev-kunit-convert-v2-1-182db9bd56ec@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>