Use memdup_user_nul() helper instead of open-coding to
simplify the code.
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Link: https://lore.kernel.org/r/20230725120247.509422-1-yangyingliang@huawei.comSigned-off-by: Mark Brown <broonie@kernel.org>