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

v1-->v2:
   fixed patch title error
v2-->v3:
   return the actual error
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zou Wei <zou_wei@huawei.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>