Reuse existing functionality from memdup_user() instead of keeping
duplicate source code.

This issue was detected by using the Coccinelle software.
Signed-off-by: NMarkus Elfring <elfring@users.sourceforge.net>
Reviewed-by: NBrijesh Singh <brijesh.singh@amd.com>
Acked-by: NGary R Hook <gary.hook@amd.com>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>