dog/vdi: bug fix for voting majority, add test case to test it

There is a critical error in the function vote_majority_object.
When all the three copies are different, the original logic still
choose the last one as majority.
The root cause is step 2 of the voting algorithm was missed.

Signed-off-by: Ruoyu <liangry@ucweb.com>
Reviewed-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>