Use kmemdup rather than duplicating its implementation

Link: https://lore.kernel.org/r/20190831124424.18642-1-yuehaibing@huawei.comSigned-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>