rbd: make get_lock_owner_info() return a single locker or NULL

Make the "num_lockers can be only 0 or 1" assumption explicit and
simplify the API by getting rid of output parameters in preparation
for calling get_lock_owner_info() twice before blocklisting.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Dongsheng Yang <dongsheng.yang@easystack.cn>