Use kmemdup_nul() helper instead of open-coding to simplify the code.

Link: https://lore.kernel.org/r/20210402092517.2445595-1-yangyingliang@huawei.comReported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>