vmemdup_user is better than duplicating its implementation, So just replace
the open code.

The issue is detected with the help of Coccinelle.
Tested-by: Don Brace <don.brace@microsemi.com>
Acked-by: Don Brace <don.brace@microsemi.com>
Signed-off-by: zhong jiang <zhongjiang@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>