Use kobj_to_dev() instead of open-coding it.

Link: https://lore.kernel.org/r/20220510105113.1351891-1-chi.minghao@zte.com.cnReported-by: Zeal Robot <zealci@zte.com.cn>
Acked-by: Brian King <brking@linux.vnet.ibm.com>
Signed-off-by: Minghao Chi <chi.minghao@zte.com.cn>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>